Among the many mysteries inside Iran's ruling hierarchy, the Quds Force has a special place in the shadows.
It's been linked by Western officials and others to dozens of clandestine operations around the world such as a deadly bombing of a Jewish cultural center in Buenos Aires in 1994, aiding Shiite militias in Iraq and helping arm Afghanistan's Taliban _ and now as the alleged masterminds of a plot to kill the Saudi Arabian ambassador in Washington.
Imagine the spymaster cloak of the CIA, the under-the-radar capabilities of military Special Forces and the deep-pocket resources of the Pentagon and big business rolled together. This comes close to explaining the powerful underpinnings of the Quds Force, the Arabic word for Jerusalem and a reference to the city's Islamic holy sites.
The Quds Force _ with between 5,000 and 15,000 agents and field tacticians by various estimates _ sits atop the vast military and industrial network of the Revolutionary Guard, the defenders of Iran's ruling clerics and their hold on power.
The Guard effectively has a blank check. It controls most major programs _ including nuclear, missile development and Iran's budding space efforts _ as well as a millions-strong paramilitary corps known as the Basiji that's been used as street muscle to put down protests.
The Quds Force is seen as the Guard's A-team around the world. The favored route, experts say, is the low-risk channels of arming and training proxies such as Hezbollah in Lebanon, Hamas in Gaza and Shiite militiamen in Iraq _ whose armor-piercing roadside bombs have been linked by the U.S. military to Iran.
In July, the U.S. military's top spokesman in Iraq, Maj. Gen. Jeffrey S. Buchanan, attributed a sharp rise in attacks to a suspected Quds-aided faction known as the Hezbollah Brigades. Earlier this year, Western intelligence officials in Afghanistan claimed a Taliban leader met in Iran with Quds Force personnel to ask for stronger weapons to fight NATO forces.
A 2007 report by the Washington-based Center for Strategic and International Studies claimed that Quds agents have special "sections" in Iranian embassies that are off limits to regular diplomatic staff. It's unclear, the report said, whether even the ambassadors have full knowledge of Quds Force operations.
"Its ranks are said to be comprised of Iran's most highly skilled special operations and intelligence officers," said Michael S. Smith II, a counterterrorism expert and co-founder of the security consultant group Kronos Advisory, which presented a report on the Quds Force to a congressional caucus in April.
The report described the Quds Force as part of a "known unknown" for Western security officials trying to track its network.
In August, the European Union announced imposed harsher sanctions against the Quds Force, saying it had given support to Syrian President Bashar Assad _ a key Iranian ally _ in attacks against anti-government protesters. The U.S. Treasury in 2007 declared the Quds Force a "specially designated global terrorist organization."
Iran, however, barely acknowledges the Quds Force exists. The group is not mentioned in the national budget and doesn't openly participate in military parades alongside its Revolutionary Guard partners. A former CIA officer, Robert Baer, has said it's believed that the Quds Force requires the use of couriers for all sensitive communications _ a technique that was also used by Osama bin Laden.
One of the few public faces with known Quds links is Defense Minister Gen. Ahmad Vahidi, who was the force's commander during the 1994 bombing of a Jewish cultural center in Buenos Aires that killed 85 people. Vahidi and four others are wanted by Argentina in connection with the attack.
Quds commanders, however, come under no public scrutiny in Iran and have an open door to Supreme Leader Ayatollah Ali Khamenei, who has the last word in all major affairs.
If this week's U.S. allegations prove true, a Quds plot would potentially have links to the highest levels of Iran's theocracy.
While the U.S. has no direct proof, and did not charge in court, that the top Iranian leaders approved an attack on Saudi envoy Adel Al-Jubeir, a U.S. official said any such operation would be vetted at the highest levels.
President Barack Obama said Thursday that the U.S. will make sure that Iranian officials are held accountable for "reckless behavior." People in the Iranian government "were aware of this plot," Obama told a White House news conference.
Iran, in turn, has called the U.S. allegations baseless.
Manssor Arbabsiar, a 56-year-old U.S. citizen who also holds an Iranian passport, was charged along with Gholam Shakuri, who authorities said was a Quds Force member and is still at large in Iran.
U.S. officials described the assassination plot as "amateur hour."
The sloppy and traceable aspects of the alleged conspiracy _ such as two normal bank transfers of more than $100,000 and bringing an Iranian-American used car salesman into the plot _ appear contrary to the Quds' hallmarks of working through third parties and carefully avoiding leaving fingerprints.
Rolf Tophoven, director of the Institute for Terrorism Research and Security Policy in Essen, Germany, said the U.S. accusations raised "a lot of questions."
"I'm very skeptical," he said. "If the government of Iran wanted to do something against a foreign Arab diplomat, it would not be necessary to do it in the United States. They could do it in any country of the Middle East."
Tophoven said a possible scenario is that "radical elements" of Quds could have acted on their own.
"Maybe some hard-core elements in Quds did it on their own, not on the order of the government," he said. "These guys (the Iranian government) are not so stupid to give an order to kill a foreign diplomat because the damage to the regime in Tehran would be catastrophic in the Middle East and elsewhere."
Like most of Iran's present military system, the Quds Force took shape during the 1980-88 war with Saddam Hussein's Iraq _ which at that time was backed by Washington.
Some analysts see Quds Force's hands emerging as early as the 1983 truck bombings of U.S. Marine barracks in Beirut that killed 241 Americans. Others claim the Quds Force helped direct the 1996 bombings of the Khobar Towers in Saudi Arabia that killed 19 American military personnel.
Any possible clues of a Quds Force role are long buried under layers of front organizations and the meticulous secrecy that has become the group's hallmark.
On the battlefield, however, it is harder to hide involvement. Iran has supplied Hezbollah with Fajr-4 and Fajr-5 rockets used in the 2006 summer war with Israel. In Iraq, suspected Iranian-linked roadside bombs, known as IEDs, were once the chief killer of U.S. troops.
"What we do know is that the Quds Force was instrumental in providing these deadly IEDs to networks inside of Iraq," said then U.S. President George W. Bush in February 2007. "And we also know that the Quds Force is a part of the Iranian government. That's a known. What we don't know is whether or not the head leaders of Iran ordered the Quds Force to do what they did."
Associated Press writer David Rising in Berlin contributed to this report.