The Quds force, meaning Jerusalem force, is, in effect, a group of secret agents and hit men who answer only to the religious authority of the Iranian government.
"The Quds force is a part of the Iranian government. Whether Ahmadinejad ordered the Quds force to do this, I don't think we know," President Bush said today in his live press conference.
What U.S. officials do know, and have known for years, is that the Quds force is under the control of the country's Grand Ayatollah Ali Khamenei, not Iran's president.
"Quds force reports directly to the Supreme Ayatollah, through the commander-in-chief of the revolutionary guards," Richard Clarke, a former U.S. counterterrorism official and now ABC News consultant, explained. "So it is possible that the ministers in the Iranian government don't know what the Quds force is doing."
The U.S. says it has conclusive evidence that the Quds force is moving lethal roadside bombs into Iraq that killed at least 170 U.S. troops.
And U.S. officials say Quds force members have been taken into custody in Iraq, suspected of funding and training Shiite militias.
"They are the trained killers; they are the special forces," Clarke told ABC News.
But less clear, as President Bush conceded today, is who in the Iranian government may have sent them.
"I don't think we know who picked up the phone and said to the Quds force, 'Go do this,'" Bush said.
But U.S. officials say there's no doubt that the Quds force has long had American blood on its hands, linking them to the 1983 attacks in Beirut on the U.S. embassy and marine barracks, as well as the bombing of a Jewish community center in Buenos Aires, and a series of assassinations of Iranian dissidents in Europe.
"The cream of the fundamentalist crop are the people who really get recruited into the Quds Force," Roya Hakakian of the Whitney Humanities Center at Yale University explained.
In a series of recent speeches, the commander overseeing the Quds force spoke of American forces in Iraq sunk in a quagmire and highly vulnerable, with Iranian strategy on U.S. weaknesses.