The U.S. military held back from using its most powerful bunker-buster bombs on Iran’s heavily fortified Isfahan nuclear site last weekend because the facility is buried so deep underground that the weapons would likely have been ineffective, according to Joint Chiefs of Staff Chairman Gen. Dan Caine.
Caine’s explanation, shared during a closed-door classified briefing with senators on Thursday, is the first public insight into why the Pentagon didn’t deploy the Massive Ordnance Penetrator (MOP) on the site believed to house nearly 60% of Iran’s enriched uranium. Three people present at the briefing and a fourth briefed afterward confirmed his remarks.
Instead, the U.S. struck Isfahan with Tomahawk missiles launched from a submarine, while Fordow and Natanz — other key Iranian nuclear sites — were targeted with more than a dozen bunker-buster bombs dropped by B-2 bombers.
The high-level briefing, conducted by Caine, Defense Secretary Pete Hegseth, Secretary of State Marco Rubio, and CIA Director John Ratcliffe, offered lawmakers a clearer picture of the operation and its limits. A spokesperson for Caine declined to comment on the classified session.
Ratcliffe told lawmakers the intelligence community believes Iran’s largest nuclear material stockpiles are located at the Isfahan and Fordow sites. Democratic Sen. Chris Murphy later told CNN that some of Iran’s nuclear infrastructure is “so far underground that we can never reach them,” acknowledging the challenges in fully neutralizing Tehran’s capabilities.
A preliminary assessment from the Defense Intelligence Agency (DIA), produced the day after the strikes, concluded that the attack did not destroy the core of Iran’s nuclear program — including its enriched uranium — and likely only delayed Iran’s progress by a few months. The report also raised the possibility that Iran had moved some enriched material prior to the attacks.
While President Trump has publicly insisted nothing was moved from the sites, Republican lawmakers acknowledged the military wasn’t aiming to destroy Iran’s entire nuclear stockpile.
“There is enriched uranium that moves around those facilities, but that wasn’t the mission,” said Rep. Michael McCaul (R-TX). “We need a full accounting. That’s why Iran has to come to the table so the IAEA can verify what’s still there.”
Rep. Greg Murphy (R-NC) echoed that point: “The purpose of the mission was to eliminate specific parts of the program — not to remove the nuclear material itself.”
Sen. Lindsey Graham (R-SC) added, “The facilities were obliterated. Nobody can use them anytime soon. But the ambition is still there.”
Intelligence and satellite imagery analysts say Iran may have already begun to access the Isfahan tunnels following the strike. Jeffrey Lewis, a weapons expert at the Middlebury Institute of International Studies, told CNN that satellite images showed activity at the site on June 26 and 27, including a cleared tunnel entrance.
“If Iran’s highly enriched uranium was still in that tunnel when they sealed it, it might now be somewhere else,” Lewis warned.
Images from Planet Labs confirmed that at least one tunnel entrance at the Isfahan Nuclear Technology Research Center was open as of June 27.
Despite the heavy damage to surface structures, U.S. officials say underground stockpiles may remain untouched. Sources told CNN the destruction above ground could at least make it harder for Iran to access or move its remaining nuclear material — though that would only be a temporary setback.
Sen. Murphy concluded Thursday that while the strikes were effective in damaging infrastructure, Iran still has the knowledge and tools to resume its nuclear program. “If they still have the enriched material and the centrifuges, and they can quickly reassemble them, we haven’t set their program back by years — only months.”
Officials said the attack on Fordow was executed exactly as planned but gave no detailed assessment of the aftermath at Isfahan or Natanz.
