– New details from a trove of Iranian nuclear documents stolen by Israeli spies early this year show that Tehran obtained explicit weapons-design information from a foreign source and was on the cusp of mastering key bombmaking technologies when the research was ordered halted 15 years ago.

Iran's ambitious, highly secretive effort to build nuclear weapons included extensive research in making uranium metal as well as advanced testing of equipment used to generate neutrons to start a nuclear chain reaction, the documents show.

While Iranian officials halted much of the work in 2003, internal memos show senior scientists making extensive plans to continue several projects in secret, hidden within existing military research programs.

"The work would be divided in two: covert (secret structure and goals) and overt," an Iranian scientist writes in one memo, part of a 100,000-document archive seized in a daring raid on a storage facility in Tehran by Israel's Mossad intelligence agency in January.

The stolen documents contain no revelations about recent nuclear activity and no proof that Iran has violated the 2015 nuclear accord it reached with the United States and five other global powers. U.S. officials had long known of Iran's pre-2004 nuclear weapons research, which the Obama administration cited explicitly in prodding Iran to accept the historic deal limiting its ability to make enriched uranium and placing its nuclear facilities under intensive international oversight.

Israeli Prime Minister Benjamin Netanyahu has seized on the documents in recent weeks to launch new attacks against the nuclear deal, which Israeli officials say is inadequate for containing Iran's long-term nuclear ambitions. The accord has been on life support since the Trump administration unilaterally withdrew from the pact in May. Iran says it is honoring the terms of the agreement and has no intention of building nuclear weapons.

A large team of Israeli experts has continued to mine the document trove for new revelations while simultaneously sharing the material with U.S. and European intelligence agencies as well as with the International Atomic Energy Agency, or IAEA, the U.N. watchdog in charge of monitoring Iran's nuclear activity. Officials shared recent discoveries with a small group of Western news outlets last week, arguing that the newly uncovered evidence of Tehran's advanced nuclear weapons research — along with its elaborate efforts to conceal the activity while preserving the technical know-how for possible future use — shows that Iran cannot be trusted. Iran has disputed the authenticity of the documents obtained by Israel, calling them forgeries. Officials at Iran's U.N. mission in New York did not reply to a request for comment.

Many U.S.-based weapons experts and former U.S. officials say the new revelations show precisely why the nuclear deal was necessary.

"We were at the table precisely because we knew that Iran harbored ambitions to build a nuclear bomb, and we wanted a verifiable agreement to block those ambitions," said Jake Sullivan, a former State Department official involved in early discussions with Iran over what would later become the Joint Comprehensive Plan of Action, or JCPOA, as the nuclear deal is commonly known.