A key player in the Iran-backed plot to assassinate Saudi Arabia's ambassador to the U.S. was a senior military commander linked to the slaughter of U.S. troops in Iraq, The Washington Post reported Saturday.

Abdul Reza Shahlai is the cousin of accused plotter Mansour Arbabsiar, 56, an Iranian-American currently in custody and charged with a string of offenses including conspiracy to commit murder and an act of international terrorism.

Along with a co-conspirator who remains at large in Iran, Arbabsiar -- a used car salesman based in Corpus Christi, Texas -- allegedly planned to bomb a Washington, D.C. restaurant where he believed Saudi Ambassador Adel al Jubeir was a regular customer.

The plotters also allegedly discussed bombing the Saudi and Israeli embassies in Washington and the Israeli embassy in Argentina.

After the details of the foiled plan were announced Tuesday, the U.S. Treasury Department moved to block the assets of five individuals believed to be directly linked to the conspiracy.

One of them was Shahlai.

The 54-year-old is a commander in Iran's Quds Force, the body believed to have been behind the Saudi ambassador plot and described to the Post by a US official as "Iran's arm for supporting terrorists and planning attacks."

In 2007 Shahlai ran a group of elite killers within the Iraqi militia of the cleric Moqtada al Sadr, who dressed as US and Iraqi soldiers and launched an attack on official buildings in Karbala -- a raid which left five Americans dead.

He also supplied al Sadr's group with weaponry, according to a Treasury report cited by the Post.

He was allegedly dealing with Arbabsiar when the Texas-based salesman returned to Iran earlier this year, hoping to use him as a link to Mexican drug traffickers who would be involved in the assassination plot.

"The Quds Force ... has, in the past, reached out to groups that might seem unlikely partners," a U.S. official told the newspaper. "The U.S. government has known for quite some time that the Quds Force was involved in this type of external plotting and has known that Shahlai has been behind much of it. That he is still at it is no surprise."