The alleged plot to target the Saudi ambassador to the U.S. while in Washington, D.C. was allegedly run by the secretive military group called the Quds Force.
The Quds Force is a sort of subdivision of the Iranian National Guard thought to number 15,000 men. It has a specific mission: To support and train terrorists and insurgents outside Iran.
We have no pictures of them, and that's the point. They're a shadowy, secretive force, but we can see the results of some of their operations.
In 1994, a bomb destroyed the Jewish community center in Buenos Aires in Argentina, killing 85 people.
Evidence suggested the attack was planned and executed by the Quds force.
In 2007, deadly explosive charges started showing up in Iraq, so sophisticated that they were able to pierce America's extra-armored vehicles.Holder: Iran aimed to bomb Saudi ambassador
President Bush himself pointed the finger at the Quds force.
"I can say with certainty that the Quds force, a part of the Iranian government, has provided these sophisticated IEDs that have harmed our troops," Mr. Bush said at the time.
This shadowy unit is led by Brigadier General Qassem Soleimani, who was designated a supporter of terrorism by the U.S. State Department in 2007.
He doesn't answer to any elected official, but to Iran's chief cleric, Supreme Leader Ali Khamenei.
His force, which is believed to fund itself in part through smuggling, also supports the militant armed wing of Hezbollah in Lebanon and Hamas in Gaza.
People who watch the Quds Force closely say that the alleged attack plot in Washington is unusually brazen for them and represents an unprecedented provocation.