This courtroom sketch by Christine Cornell shows Iranian-American citizen Manssor Arbabsiar, center, as he pleads guilty Wednesday at New York Federal Court to plotting with the Iranian military to kill the Saudi ambassador to United States. (CHRISTINE CORNELL/AFP/GETTY IMAGES)

A former used-car salesman accused of conspiring with Iranians in an audacious murder-for-hire plot pleaded guilty Wednesday to helping plan the assassination of a Saudi diplomat at a posh Georgetown restaurant.

Manssor Arbabsiar, 58, a Texan with dual Iranian and U.S. citizenship, entered the plea in a New York courtroom just over year after his arrest in a case that shocked the world and drove U.S.-Iranian relations to a new low.

Arbabsiar faces up to 25 years in prison for his role in the plot, which U.S. intelligence officials say was part of an unusually brazen attempt by senior Iranian military officials to carry out an act of terrorism on U.S. soil. The scheme unraveled when Arbabsiar attempted to hire a Mexican hit man who turned out to be an undercover informant working for U.S. counternarcotics agents.

“This case underscores the evolving threat environment we face,” said Lisa Monaco, the Justice Department’s assistant attorney general for national security. Despite initial skepticism about Tehran’s sponsorship, evidence bore out U.S. claims of an “international assassination plot hatched in Iran,” she said.

The assassination scheme garnered international attention because it was the first known instance of Iranian government officials reaching deep inside U.S. territory in an alleged attempt to kill a high-ranking foreign diplomat — in this case, Adel al-Jubeir, the Saudi ambassador to the United States. U.S. prosecutors, citing phone intercepts and other evidence, allege that the plan originated within an elite Iranian military unit known as the Quds Force and had high-level Iranian approval.

Arbabsiar was tasked by his Iranian cousin, a Quds Force officer identified by U.S. officials as Gholam Shakuri, to find accomplices to help kill Jubeir as he dined at Café Milano, an upscale eatery in the District’s Georgetown section popular with politicians and diplomats, according to law enforcement officials and court documents. Shakuri, who was indicted along with Arbabsiar, remains at large in Iran.

The Texan was arrested after wiring a $100,000 down payment to the undercover informant, a man he believed was a member of a Mexican drug cartel. After his capture, Arbabsiar allowed U.S. officials to listen in on his conversations with his Iranian contacts.

Western intelligence agencies would eventually link the plot to a series of Iranian-backed attempts to attack Israeli, Saudi and Western targets in at least six countries around the world over the past year. Analysts and counterterrorrism experts believe the plots are part of an ongoing covert war waged by Iran in retaliation for attacks on Iranian scientists and nuclear facilities.

Though badly bungled, the attempt to strike targets in Washington suggested that top Iranian leaders had “changed their calculus” on whether to risk direct attacks in the United States, said Matthew Levitt, director of the Stein Program on Counterterrorism and Intelligence at the Washington Institute for Near East Policy, a think tank.

“This is less about Arbabsiar and more about the Quds Force and how it operates,” he said. “As recent events have made clear, Iran is willing now to strike not only Israeli targets but U.S. targets as well.”