The Israeli attack on Iran in late October destroyed an active top secret nuclear weapons research facility in Parchin, according to three U.S. officials, one current Israeli official and one former Israeli official.
Why it matters: The strike — which targeted a site previously reported to be inactive — significantly damaged Iran's effort over the past year to resume nuclear weapons research, Israeli and U.S. officials said.
One former Israeli official briefed on the strike said it destroyed sophisticated equipment used to design the plastic explosives that surround uranium in a nuclear device and are needed to detonate it.
Iran has denied it is pursuing nuclear weapons. Iranian Foreign Minister Abbas Araghchi said in a statement last week that "Iran is not after nuclear weapons, period."
The Iranian mission to the UN declined to comment for this story.
The incoming Trump administration will include several key national security and foreign policy officials who are hawkish on Iran, which could lead to increased U.S. pressure on the Islamic Republic.
Driving the news: One of the targets of the Israeli strike on Oct. 25 was the Taleghan 2 facility in the Parchin military complex, about 20 miles southeast of Tehran.
The facility was part of the Iranian Amad nuclear weapons program until Iran halted its military nuclear program in 2003. It was used for testing explosives needed to set off a nuclear device, according to the Institute for Science and International Security.
High-resolution satellite imagery acquired by the institute after the Israeli strike showed the Taleghan 2 building was completely destroyed.
Behind the scenes: Israeli and U.S. officials said the activity that took place recently at the Taleghan 2 facility was part of an effort inside the Iranian government to conduct research that could be used for the development of nuclear weapons but could also be presented as research for civilian purposes.
"They conducted scientific activity that could lay the ground for the production of a nuclear weapon. It was a top secret thing. A small part of the Iranian government knew about this, but most of the Iranian government didn't," a U.S. official said.
Israeli and U.S. intelligence began detecting research activity at Parchin earlier this year, including Iranian scientists conducting computer modeling, metallurgy and explosive research that could be used for nuclear weapons.
Flashback: Last June, the White House officials privately warned the Iranians in direct conversations about the suspicious research activities, Axios reported.
The U.S. hoped the warning would make the Iranians stop their nuclear activity, but they continued, the officials said.
A U.S. official said that in the months before the Israeli attack "there was concern across the board" about the Iranian activity at the Taleghan 2 facility.
The Iranian nuclear weapons research even led the U.S. Director of National Intelligence (DNI) to change its assessment about the Iranian nuclear program.
The Wall Street Journal reported in August that a DNI report to Congress didn't include a sentence that had appeared in intelligence assessments in recent years, which said Iran "is not currently undertaking the key nuclear weapons-development activities necessary to produce a testable nuclear device."
Between the lines: When Israel prepared its retaliation for Iran's massive Oct. 1 missile attack, the Taleghan 2 facility was chosen as a target.
President Biden asked Israeli Prime Minister Benjamin Netanyahu not to attack the Iranian nuclear facilities in order not to trigger a war with Iran, U.S. officials said.
But Taleghan 2 was not part of Iran's declared nuclear program so the Iranians wouldn't be able to acknowledge the significance of the attack without admitting they violated the nuclear non-proliferation treaty.
"The strike was a not so subtle message that the Israelis have significant insight into the Iranian system even when it comes to things that were kept top secret and known to a very small group of people in the Iranian government," a U.S. official said.
What to watch: The International Atomic Energy Agency (IAEA) board of governors will meet next week and is expected to vote on a censure resolution against Iran for its lack of cooperation with the UN nuclear watchdog. Iran has said it could respond by limiting its cooperation with the IAEA.
IAEA Director General Rafael Grossi visited Iran on Wednesday and Thursday ahead of the board meeting.
Araghchi told Grossi that Iran is willing to negotiate with the E3 — France, Germany and the UK — "on its peaceful nuclear program ... but not ready to negotiate under pressure and intimidation."