Recounting the steps that led to his breaking the story, Mr. Sheehan told of aliases scribbled into the guest registers of Massachusetts motels; copy-shop machines crashing under the burden of an all-night, purloined-document load; photocopied pages stashed in a bus-station locker; bundles belted into a seat on a flight from Boston; and telltale initials incinerated in a diplomat’s barbecue set.

He also revealed that he had defied the explicit instructions of his confidential source, whom others later identified as Daniel Ellsberg, a former Defense Department analyst who had been a contributor to the secret history while working for the Rand Corporation. In 1969, Mr. Ellsberg had illicitly copied the entire report, hoping that making it public would hasten an end to a war he had come passionately to oppose.

Contrary to what is generally believed, Mr. Ellsberg never “gave” the papers to The Times, Mr. Sheehan emphatically said. Mr. Ellsberg told Mr. Sheehan that he could read them but not make copies. So Mr. Sheehan smuggled the papers out of the apartment in Cambridge, Mass., where Mr. Ellsberg had stashed them; then he copied them illicitly, just as Mr. Ellsberg had done, and took them to The Times.

Over the next two months, he strung Mr. Ellsberg along. He told him that his editors were deliberating about how best to present the material, and he professed to have been sidetracked by other assignments. In fact, he was holed up in a hotel room in midtown Manhattan with the documents and a rapidly expanding team of Times editors and reporters working feverishly toward publication.

The publication of the first installment of the Pentagon papers on June 13, 1971, blindsided Mr. Ellsberg. He learned it was imminent from another Times staff member, Anthony Austin, with whom he had secretly shared an excerpt months before. Mr. Austin had chosen not to mention the bombshell to anyone at the newspaper, preferring to keep it for a book he was writing about the war.

When Mr. Austin discovered that his own newspaper was about to scoop his scoop, he called Mr. Ellsberg in a panic. Mr. Ellsberg tried to reach Mr. Sheehan, who was on deadline writing a subsequent installment. Mr. Sheehan ignored Mr. Ellsberg’s messages until he knew it would be too late in the press run to intervene. He asked an editor to let him know when 10,000 copies had been printed.

“You had to do what I did,” Mr. Sheehan said in the 2015 interview, justifying his deception of Mr. Ellsberg, whom he described as torn between his desire to make the papers public and his fear of being sent to prison. In his efforts to protect himself, Mr. Sheehan said, Mr. Ellsberg was behaving recklessly. Mr. Sheehan feared that Mr. Ellsberg would inadvertently tip someone off. “It was just luck that he didn’t get the whistle blown on the whole damn thing,” he said.

Fearing Prison

Mr. Ellsberg had been a source for Mr. Sheehan before. So on a visit to Washington in March 1971, Mr. Ellsberg called him and asked to spend the night at his house. During a long night of talking, the two men made a deal. As Mr. Sheehan told it, Mr. Ellsberg would give him the papers; and, if The Times agreed to publish them, the newspaper would do its best to protect the identity of its source.

But when Mr. Sheehan arrived in Cambridge intending to collect the documents, he recalled, Mr. Ellsberg had changed his mind. He told Mr. Sheehan he could read them but make no copies — because, as Mr. Sheehan described it, “once he turned loose of it, The Times would assume ownership of it, and they’d do what they wanted with it.”

“He’d lose control.”

In his 2002 memoir, “Secrets: A Memoir of Vietnam and the Pentagon Papers,” Mr. Ellsberg wrote that he was skeptical that The Times would publish the documents in full, as he had wanted. He feared, too, he added, that if he handed over the papers before The Times had committed to publishing, someone there would inform the Federal Bureau of Investigation, “or the bureau would somehow get wind of it and come after my other copies.”

To Mr. Sheehan, however, it seemed that Mr. Ellsberg’s reservations were “about going to jail.”

“Because when The Times got it,” he said, “The Times would go ahead with it. And when it came out, he might get caught. And he didn’t have a politician yet to protect him.”

He was, Mr. Sheehan said, “totally conflicted.”

Mr. Ellsberg was also taking serious risks, Mr. Sheehan said. He had made multiple copies and had paid carelessly with personal checks. He had approached members of Congress about holding hearings. “There’s no way The Times can protect this guy,” Mr. Sheehan remembered thinking. His ostensibly secret source had “left tracks on the ceiling, on the walls, everywhere,” he said.

“Sooner or later, I was afraid he was going to run into a politician who’d go right to the Justice Department,” Mr. Sheehan said. That person would get on the phone to the attorney general “and say, ‘Hey, The New York Times has got some kind of big secret study, they got it from Dan Ellsberg.’ ”

Mr. Sheehan realized, he said, that he had to move fast. Once word leaked out, the government would go to court to block publication. Lawyers for The Times would end up arguing with the Justice Department over classified material, the importance of which neither the judge nor the public would be in a position to understand.

“Oh, I felt really quite angry,” Mr. Sheehan recalled. Like Mr. Ellsberg, he had turned against the war and intended to do what he could to stop it. “So I was quite upset when Ellsberg said, ‘You can read, take notes, but no copies,’ ” he remembered. “And over the fact that he was out of control.”

He made up his mind, he said, “that this material is never again going in a government safe.”

Back in Washington, he confided in his wife, Susan Sheehan, a writer for The New Yorker. He recalled her saying, “If I were you, I’d get control of that situation.” Play along with Mr. Ellsberg, do your best to protect him, but get the material to The Times.

“Xerox it,” he remembered Ms. Sheehan saying.

He returned to Cambridge to continue reading and taking notes. When Mr. Ellsberg let it be known that he was leaving on a brief vacation, Mr. Sheehan asked to continue working in the apartment where the documents were kept. Mr. Ellsberg agreed and gave him a key. He reminded Mr. Sheehan: No copies.

Mr. Sheehan said nothing.

“I’d known Ellsberg for a long time, and he thought I was operating under the same rules that one normally used: Source controls the material,” Mr. Sheehan said. “He didn’t realize that I had decided: ‘This guy is just impossible. You can’t leave it in his hands. It’s too important and it’s too dangerous.’ ”

Long Night in a Copy Shop

When it was clear that Mr. Ellsberg was leaving, Mr. Sheehan called home. “Come up,” he told his wife. “I need your help.” He told her to bring suitcases, large envelopes and all the cash in the house. She flew to Boston and checked into a hotel under a false name. Mr. Sheehan was in a motor inn, under yet another name.

From the Times bureau chief in Boston, he got the name of a copy shop that could handle thousands of pages. He asked the bureau chief to get him several hundred dollars in expense money for a secret project he declined to explain. When the bureau chief called the Times newsroom and reached the editors on duty that night, they declined the request. So he called the national editor at home.

“Give it to him,” the editor said, according to Mr. Sheehan. No questions asked.

Mr. Sheehan duplicated the apartment key in case he lost the original. Then he began copying the seven thousand pages — first in a real estate office where an acquaintance worked, then, with Ms. Sheehan’s help, in the suburban copy shop. He was ferrying piles of pages by taxi between the apartment and the copy shop, then to a locker in the Boston bus terminal and later to a locker at Logan airport.

When the machines in the copy shop crashed under the strain, the Sheehans relocated to a copy shop in Boston run by a Navy veteran. When the man noticed that the documents were classified, and became nervous, Ms. Sheehan, at the shop, called her husband at the apartment.

“Get down here,” he remembered her saying.

He rushed back and told the manager that he had borrowed the material from some Harvard professors. They were using them for a study, he said, and had put a time limit on the loan. The documents, he assured the manager, had been declassified in bulk. The manager, being ex-Navy, seemed to understand.

At the airport, the Sheehans bought an extra seat on their flight home and piled their suitcases onto it, buckling them in rather than letting them out of their sight.

Back in Washington, Mr. Sheehan’s editor, with sample documents and a memo from Mr. Sheehan, set off for New York to get approval for Mr. Sheehan to proceed.

Mr. Sheehan and an editor planted themselves in a room at the Jefferson Hotel in Washington. They spent several weeks there reading the documents and summarizing what they had. Then they were summoned to New York to brief the newspaper’s top editors. Meeting at The Times’s headquarters on West 43rd Street, Mr. Sheehan found the company’s lawyer appearing shaken.

“It was like somebody had thrown a bucket of ice water over the man,” Mr. Sheehan remembered. “He was just terrified of what the hell I was saying. He kept saying: ‘Don’t tell them this. They won’t be able to keep the secret. Somebody will talk about it. We may have committed a felony.’ ”

He and the editor were assigned a room at the Hilton hotel in midtown Manhattan to continue working. Soon there was another editor, three more writers, security guards and file cabinets with combination locks. Eventually there were dozens of people working round the clock in three adjoining rooms. “We mapped the whole thing out,” Mr. Sheehan remembered. “And we started cranking away.”

He made a practice of calling Mr. Ellsberg every few days — “to try to keep him on the ranch,” as he put it in 2015. Mr. Sheehan was not worried about another newspaper breaking the story, he said; he was worried that someone whom Mr. Ellsberg had spoken to would blow the whistle before The Times could publish.

So he made excuses to Mr. Ellsberg for his seeming lack of progress. He said the top editors were still discussing how best to proceed. He even went up to Cambridge, he remembered, as though to take more notes. Mr. Ellsberg railed at him there, Mr. Sheehan said. “I’m taking all the risks,” he remembered Mr. Ellsberg saying. “You people aren’t taking any risk.”

A Signal, and Then a Go

A few weeks before publication, Mr. Sheehan decided to send Mr. Ellsberg a signal. He was not willing to tell him directly that The Times was going ahead because he feared that Mr. Ellsberg’s reaction might inadvertently tip the government off. But he wanted some kind of “tacit consent” from Mr. Ellsberg, he remembered.

“It was a matter of conscience,” he said.

So he told Mr. Ellsberg that he now needed the documents, not just his notes. Mr. Ellsberg had said that he would hand them over only when he was ready, knowing that The Times would then do as it pleased. This time, when Mr. Sheehan asked, Mr. Ellsberg consented.

Mr. Rosenthal congratulating reporters after publication of the Pentagon Papers in June 1971. Mr. Sheehan was second from right.Credit...Renato Perez/The New York Times

Mr. Sheehan chose to believe that the consent meant that Mr. Ellsberg understood that The Times could now publish any day.

“This was an exercise in giving Ellsberg some warning — if he remembered what he’d told me — and a bit of conscience-salving on my part,” Mr. Sheehan recalled. “Maybe it’s hypocritical, but we were going to go to press, and I wanted to try to give him some kind of warning.”

Mr. Ellsberg, it would turn out, had missed the signal.

Meanwhile, he arranged for Mr. Sheehan to pick up a complete copy of the historical study stowed in an Ellsberg family apartment in Manhattan. Mr. Sheehan remembered paying the doorman “the kind of generous tip that leads people to say, ‘I don’t know nuthin’.’ Because I knew sooner or later the F.B.I. would be trying to piece all of this together.”

He took other steps at the last minute to cover his tracks. A copy stored at the Sheehans’ house went into a colleague’s freezer. Pages of other copies bearing Mr. Ellsberg’s initials were pulped in New Jersey or burned in the barbecue set of a diplomat from Brazil, a friend of Mr. Sheehan’s father-in-law.

In the end, the timing of the publication of the Pentagon Papers took Mr. Ellsberg by surprise. When Mr. Sheehan finally returned Mr. Ellsberg’s calls, he reached only Mr. Ellsberg’s wife, who, he said, told him that Mr. Ellsberg was happy with the presentation of the material but, as Mr. Sheehan put it, “unhappy over the monumental duplicity.”

In the interview in 2015, Mr. Sheehan said he had never revealed Mr. Ellsberg’s identity while the project was underway. To his editors he always spoke only of “the sources.” It was another journalist, outside the paper, who blew Mr. Ellsberg’s cover not long after the Pentagon Papers story broke.

Nor did Mr. Sheehan ever speak about how he had obtained the papers. In 2015, he said he had never wanted to contradict Mr. Ellsberg’s account or embarrass him by describing Mr. Ellsberg’s behavior and state of mind at the time.

There was no contact between the two men for six months. Shortly before Christmas 1971, Mr. Sheehan said, they ran into each other in Manhattan. In a brief conversation, he said, he told Mr. Ellsberg what he had done.

“So you stole it, like I did,” he recalled Mr. Ellsberg saying.

“No, Dan, I didn’t steal it,” Mr. Sheehan said he had answered. “And neither did you. Those papers are the property of the people of the United States. They paid for them with their national treasure and the blood of their sons, and they have a right to it.’”