Pentagon chief Leon Panetta said Thursday that US officials had "some ideas" who was behind the assassination of a nuclear scientist in Tehran this week, but insisted the United States was not involved.
During a meeting with soldiers at a Texas military base, Panetta said the United States was "not involved in any way, in any way with regards to the assassination that took place there."
"I'm not sure who was involved. We have some ideas as to who might be involved. But we don't know exactly who was involved," he added.
The United States had denied Iranian claims that it had a role in killing the scientist, Mostafa Ahmadi Roshan, in a car bomb attack, and on Wednesday condemned it as an act of violence.
US President Barack Obama and Israeli Prime Minister Benjamin Netanyahu earlier spoke by phone, after Iran accused the allies of plotting the killing.
A senior Israeli official said he did not know who carried out the act but added that Israel did not mourn Ahmadi Roshan's loss.
Ahmadi Roshan was a deputy director at the Natanz uranium enrichment facility, according to the website of the university he graduated from a decade ago, Sharif University.
The attack fueled speculation about possible efforts by Israel or its allies to sabotage the Iranian nuclear program, at the same time as Washington and the EU tighten sanctions designed to deter Tehran's atomic plans.