New York (CNN) -- An Iranian-American who pleaded guilty to participating in a plot to kill the Saudi ambassador to the United States was sentenced Thursday to 25 years in prison.
Attorneys for Manssor Arbabsiar, 58, wanted a 10-year sentence, but U.S. District Judge John F. Keenan agreed with the prosecution's recommendation of 25 years.
Prosecutors said Arbabsiar tried to recruit a Mexican drug cartel to bomb a Washington restaurant where Saudi Ambassador Adel Al-Jubeir dined. But the scheme unraveled when Arbabsiar's cartel contact turned out to be an undercover agent.
The judge noted that when Arbabsiar was told there would likely be a lot of casualties in the assassination plot, Arbabsiar said, "No problem. No big deal."
Keenan said that others who may wish to conspire to cause harm to the United States or its interests "must learn the lesson this will not be tolerated."
"In a case like this, deterrence is of extreme importance," the judge said.
Arrested in September 2011, Arbabsiar pleaded guilty in federal court in Manhattan in October and admitted to conspiring with members of the Iranian military in the formulation of the plot.
U.S. Attorney General Eric Holder said earlier that the plot had been endorsed by parts of the Iranian government, but Iran has denied any involvement.
Arbabsiar met "on a number of occasions in Mexico with a Drug Enforcement Administration confidential source," officials said in a federal complaint.
Posing as an associate of a sophisticated and violent international drug cartel, the source was hired by Arbabsiar and his cohorts to assassinate the ambassador, the complaint said.
Before sentencing, Arbabsiar apologized and took responsibility for actions he said he knew were wrong.
"I can't change what I did, but I have a good heart. I never hurt anyone. My mind is sometimes not in a good place," he said.
The defense request for a 10-year sentence was based, among other things, on the idea that Arbabsiar's crime was a result of untreated bipolar disorder, as argued by the defense's psychiatrist, Dr. Michael B. First, a Columbia University professor of clinical psychiatry.
But Keenan said he thought the state's psychiatrist, Dr. Gregory B. Saathoff, made the point that Arbabsiar did not have serious mental illness, and his mental state should have no effect on sentencing. Arbabsiar's age was not a factor in the sentencing because of the nature of the crime, Keenan said.
Arbabsiar's public defender, Sabrina Shroff, asked the judge if Arbabsiar could be incarcerated in a facility as close as possible to Texas, where he lived before he was arrested. Keenan said he had no objection.
CNN's Carol Cratty contributed to this report.