NEW YORK (Reuters) - The arrests of four men in a suspected plot to bomb two New York synagogues have drawn fire from critics who say U.S. law enforcement relies on informants who infiltrate extremist groups that otherwise would be incapable of mounting an attack.

Civil liberties advocates and legal scholars say the case is part of a pattern since the September 11 attacks of 2001 in which paid informants are sent to mosques where they aid and encourage disgruntled Muslim men in criminal pursuits.

“We’re concerned that it was the actions of the FBI informant that really led to the alleged plot,” said Ibrahim Hooper, a spokesman for the Council on American Islamic Relations.

Police on May 20 arrested four men who they said worked with an undercover informant for a year and were caught on video leaving what they believed were live bombs outside a pair of synagogues in the Bronx borough of New York.

The men then planned to shoot down military aircraft with a guided surface-to-air missile that, like the explosives, was deactivated and provided by the informant, authorities said.

The suspects -- James Cromitie, David Williams, Onta Williams and Laguerre Payen -- are charged with conspiracy to use weapons of mass destruction and face up to life in prison if convicted. None is believed to have links to al Qaeda.

“Why the FBI is going out to create a terrorist group just so they can then solve the crime by prosecuting the terrorist group seems a little odd,” said Michael German, who spent 16 years as an FBI special agent and now works with the American Civil Liberties Union.

He cited the fact that the men in the Bronx case had difficulty buying a pistol as evidence they needed help to do real harm.

FBI spokesman Jim Margolin said it was bureau policy not to comment on pending or previous cases and he would not comment on the use of informants generally.

“They still had the intention,” Joseph Demarest, head of the New York FBI office, said of the suspects at a recent news conference.

Andrew McCarthy, a former assistant U.S. attorney who successfully prosecuted Sheik Omar Abdel Rahman for the 1993 World Trade Center bombing plot, said informants were closely monitored and essential to preventing attacks.

“If the counterterrorism strategy you want is to prevent attacks from happening, you don’t have any choice but to infiltrate potential terrorist organizations,” McCarthy said. “Your average FBI agent from Iowa is not going to be able to ... credibly infiltrate terrorist organizations.”

BENDING THE RULES

Recent plots targeting New Jersey’s Fort Dix military base, the New York subway system and Chicago’s Sears Tower have been foiled at early stages with the help of informants and have led to criminal convictions.

In 2006, a 23-year-old Pakistani immigrant, Matin Siraj, was convicted of plotting to blow up a New York subway station after meeting with an informant nearly twice his age who was recruited by police to monitor extremist Muslims at mosques.

Siraj testified the informant had inflamed his anger toward the United States.

Another informant used in multiple cases, Mohamed Alanssi, set himself on fire outside the White House in November 2004 to protest his treatment by the FBI.

German said the public should be concerned about entrapment even if, as a defense, it typically fails to sway juries.

“It really strains credulity why the FBI chose not to use undercover agents (instead of informants), and my concern is the reason why is because they know the informants will bend the rules a lot more easily,” German said.

In the Bronx case, Payen’s court-appointed lawyer said her client was “intellectually challenged” and schizophrenic.

“They look like hapless mopes who, but for the government, wouldn’t have been involved in anything, let alone a sophisticated plot,” said Columbia University Law School professor Daniel Richman.

“The problem the government faces is the concern that a group of hapless mopes, when visited by the foreign terrorist type, will turn into very willing and effective tools.”