Photographs that Israeli officials said were stolen from Iran’s nuclear archive appear to show a giant metal chamber, in a building at the Parchin military site, built to conduct high-explosive experiments.

TEL AVIV — The Mossad agents moving in on a warehouse in a drab commercial district of Tehran knew exactly how much time they had to disable the alarms, break through two doors, cut through dozens of giant safes and get out of the city with a half-ton of secret materials: six hours and 29 minutes.

The morning shift of Iranian guards would arrive around 7 a.m., a year of surveillance of the warehouse by the Israeli spy agency had revealed, and the agents were under orders to leave before 5 a.m. to have enough time to escape. Once the Iranian custodians arrived, it would be instantly clear that someone had stolen much of the country’s clandestine nuclear archive, documenting years of work on atomic weapons, warhead designs and production plans.

The agents arrived that night, Jan. 31, with torches that burned at least 3,600 degrees, hot enough, as they knew from intelligence collected during the planning of the operation, to cut through the 32 Iranian-made safes. But they left many untouched, going first for the ones containing the black binders, which contained the most critical designs. When time was up, they fled for the border, hauling some 50,000 pages and 163 compact discs of memos, videos and plans.

In late April, Prime Minister Benjamin Netanyahu announced the results of the heist, after giving President Trump a private briefing at the White House. He said it was another reason Mr. Trump should abandon the 2015 nuclear deal, arguing that the documents proved Iranian deception and an intent to resume bomb production. A few days later, Mr. Trump followed through on his longstanding threat to pull out of the accord — a move that continues to strain relations between the United States and European allies.

Last week, at the invitation of the Israeli government, three reporters, including one from The New York Times, were shown key documents from the trove. Many confirmed what inspectors from the International Atomic Energy Agency, in report after report, had suspected: Despite Iranian insistence that its program was for peaceful purposes, the country had worked in the past to systematically assemble everything it needed to produce atomic weapons.

“It’s quite good,” Robert Kelley, a nuclear engineer and former inspector for the agency, said in Vienna, after being shown some of the fruits of the document theft. “The papers show these guys were working on nuclear bombs.”

There is no way to independently confirm the authenticity of the documents, most of which were at least 15 years old, dating from the time when an effort called Project Amad was ordered halted and some of the nuclear work moved deeper under cover. The Israelis handpicked the documents shown to the reporters, meaning that exculpatory material could have been left out. They said some material had been withheld to avoid providing intelligence to others seeking to make weapons.

The Iranians have maintained that the entire trove is fraudulent — another elaborate scheme by the Israelis to get sanctions reimposed on the country. But American and British intelligence officials, after their own review, which included comparing the documents to some they had previously obtained from spies and defectors, said they believed it was genuine.

From what the Israelis showed to the reporters in a secure intelligence facility, a few things are clear.

The Iranian program to build a nuclear weapon was almost certainly larger, more sophisticated and better organized than most suspected in 2003, when Project Amad was declared ended, according to outside nuclear experts consulted by The Times. Iran had foreign help, though Israeli officials held back any documents indicating where it came from. Much was clearly from Pakistan, but officials said other foreign experts were also involved — though they may not have been working for their governments.

The documents detailed the challenges of integrating a nuclear weapon into a warhead for the Shahab-3, an Iranian missile. One document proposed sites for possible underground nuclear tests, and described plans to build an initial batch of five weapons. None were built, possibly because the Iranians feared being caught, or because a campaign by American and Israeli intelligence agencies to sabotage the effort, with cyberattacks and disclosures of key facilities, took its toll.

The Israeli prime minister, Benjamin Netanyahu, presenting the Iranian documents in Tel Aviv in April.

David Albright, a former inspector who runs the Institute for Science and International Security, said in an interview that the documents contained “great information.”

“Iran conducted many more high-explosive tests related to nuclear weapons development than previously known,” he told Congress last month.

But the archive also shows that after a burst of activity, a political mandate…