* Defendant must prove not predisposed to commit crime
* No successful entrapment defense in big terrorism cases
WASHINGTON, Oct 13 (Reuters) - The Iranian-American charged with plotting to assassinate Saudi Arabia's top diplomat in Washington could claim he was wrongly entrapped by U.S. law enforcement, but if he does, he will likely have a hard time proving it.
Manssor Arbabsiar was arrested last month and accused of approaching someone in Mexico he thought could carry out the hit against Saudi Ambassador Adel al-Jubeir, but that man turned out to be a U.S. law enforcement snitch.
The Obama administration has conducted numerous undercover and sting operations leading to arrests of many people seeking to carry out bombing or other attacks on U.S. soil, prompting some questions about the tactics used.
No individuals charged in terrorism cases involving undercover operations since the Sept. 11, 2001, attacks have prevailed using an entrapment defense, said Georgetown University law professor David Cole, a longtime expert on the subject who has challenged terrorism policies in U.S. court.
"Entrapment requires the defendant to show that he was not predisposed to commit the crime, so that the crime was instigated by the government rather than by himself," said Cole.
"The more serious the crime, the more difficult it is to imagine that someone would commit it who was not predisposed to commit it," he said.
After several high-profile cases emerged last year, including allegations that a Somali man tried to detonate a bomb at a crowded holiday ceremony in Oregon, U.S. Attorney General Eric Holder strongly defended undercover tactics.
"These types of operations have proven to be an essential law enforcement tool in uncovering and preventing potential terror attacks," Holder said in December. In some operations, inert bombs were provided to the accused individuals.
He added such tactics "have helped identify and defuse public safety threats such as those posed by potential terrorists, drug dealers and child pornographers for decades."
A U.S. Justice Department official said there are strict guidelines for such investigations and they undergo extensive legal reviews by prosecutors and law enforcement agents. They also typically require approvals by senior officials.
BEEFED UP GUIDELINES
The U.S. government beefed up its guidelines and oversight after an embarrassing case in the 1990s involving reputed mobster James "Whitey" Bulger, an FBI informant who continued to carry out his alleged illegal activities including murders.
To ensure an individual is not entrapped, undercover agents offer the person chances to back out and typically discuss possible collateral damage, including killing bystanders.
In the case of Arbabsiar, the criminal complaint described how he approached an individual in Mexico -- who turned out to be a paid U.S. Drug Enforcement Administration informant -- with a plot to blow up a Saudi Arabian embassy.
Arbabsiar also discussed with the informant assassinating the Saudi ambassador, which was the plot they pursued, according to the sworn complaint signed by an FBI agent.
While the plot to kill Jubeir never reached the operational stage, the paid informant discussed with Arbabsiar that if they bombed a restaurant the diplomat frequented, potentially 150 others would be killed, including U.S. lawmakers.
Arbabsiar dismissed such concerns, stating "no problem" or "no big deal," according to the criminal complaint. Iran has fiercely denied the allegations of the plot to kill Jubeir.
Arbabsiar plans to plead not guilty to the charges, according to his public defender attorney. However, the sworn complaint said that after he was arrested, Arbabsiar admitted he agreed to pay the informant to carry out the assassination.
The informant, who was not identified, was paid and narcotics charges against him were dropped in return for his cooperation. He previously provided information that led to drug seizures, authorities have said.
Benjamin Wittes, a senior fellow at the Brookings Institution, said such tactics are common, have long been used, and that their "general legality is not, to my knowledge, the subject of serious question."
At the same time, a longtime counterterrorism expert and former Bush administration official, Juan Zarate, noted the Obama administration will "certainly need to present information beyond the four corners of the complaint to convince allies and rivals alike to pressure Iran further."
Another law professor raised a further risk to the case: would Washington have to reveal sources and methods in tracking the alleged plot back to Tehran.
"We're not about to reveal how we penetrate Iran's clandestine operations," said Michael Corgan, a Boston University international relations professor. "Our going public with as much as we have is mostly for propaganda purposes."
In other undercover operations, a Jordanian man was arrested in September 2009 for trying to bomb a 60-story Dallas skyscraper. The same day, an Illinois man was arrested in a similar plot and sting case. Both pleaded guilty.
The Supreme Court last weighed in on entrapment in 1992, when it overturned the conviction of a Nebraska man for receiving child pornography in the mail after finding that agents improperly excited the man's interest in the material and exerted substantial pressure on him during their lengthy investigation to obtain and read the material.