Man in Alleged Iran-Backed D.C. Assassination Plot Pleads Guilty The man accused of plotting with the Iranian military to assassinate the Saudi ambassador in Washington, D.C. pleaded guilty today. Iranian-American Manssor Arbabsiar was arrested in late September 2011 for his alleged role in what U.S. officials said at the time was a plot to commit a "significant terrorist attack in the United States." The U.S. said Arbabsiar, a 58-year-old from Corpus Christi, Texas, was working for elements of the Iranian government -- specifically Iran's elite military unit the Quds force -- when he attempted to hire hitmen from the feared Zetas Mexican drug cartel to carry out the hit on the Saudi ambassador. Arbabsiar allegedly plotted to bomb a popular D.C. restaurant frequented by the ambassador. He didn't know he'd been speaking to a DEA informant from the start. Arbabsiar originally pleaded not guilty to five counts, including conspiracy to use a weapon of mass destruction and conspiracy to murder a foreign official. READ: Criminal Complaint Against Manssor Arbabsiar (PDF) After his arrest, Iranian officials strongly rejected the U.S. accusations, calling them a "fabrication." The head of the Iranian mission to the United Nations penned a letter to U.N. Secretary General Ban Ki-moon expressing "outrage" at the allegations. "The U.S. allegation is, obviously, a politically-motivated move and a showcase of its long-standing animosity towards the Iranian nation," the letter says. Top U.S. officials, including President Obama, said weeks after the arrest that the U.S. government was confident that the allegations made against Iran could be clearly backed up by evidence. We would not be bringing forward a case unless we knew exactly how to support all the allegations... Obama said then. In a deal with the government, Arbabsiar pleaded guilty to three counts related to the murder scheme, admitted to conspiring with Iranian military officials and faces up to 25 years in prison. FBI acting assistant director Mary Galligan said Arbabsiar's reversal just confirmed what the bureau already believed about the plot. "Others who believe that they can carry out or even attempt to plan such brazen plots should be on notice: the FBI remains ever vigilant towards acts of terror both here and abroad," she said. Alleged Terror Plotter Claims He Was 'Directed By High-Ranking' Iranian Officials The federal operation to catch Arbabsiar, called Operation Red Coalition, began in May when the Iranian-American approached a DEA informant seeking the help of a Mexican drug cartel to assassinate the Saudi ambassador, counter-terrorism officials told ABC News last October. Arbabsiar thought he was dealing with a member of the feared Zetas Mexican drug organization, according to agents. The DEA office in Houston brought in FBI agents as the international terror implications of the case became apparent. Arbabsiar reportedly claimed he was being "directed by high-ranking members of the Iranian government," including a cousin who was "a member of the Iranian army but did not wear a uniform," according to a person briefed on the details of the case. The State Department has listed Iran as a "state sponsor" of terror since 1984. Officials in Argentina have said Iran was behind an attack on the Israeli embassy in Buenos Aires in 1992. ABC News' Pierre Thomas contributed to this report.