A police probe that began with a series of gas station holdups here has broadened into an investigation of a possible terrorist plot and connections to a radical Islamic group believed to be operating in the California prison system, law enforcement sources said.

The crime spree -- which police said ended with the arrest of two local men last month -- drew the interest of counterterrorism officials when a search of one suspect's home turned up "jihadist" literature, bulletproof vests and a list of addresses for area synagogues, the Israeli consulate, National Guard centers and other sites.

Now, federal officials have arrested a third man -- a Pakistani national who lived in south Los Angeles, not far from the others -- in the investigation. A law enforcement official said more arrests may follow.

Others under scrutiny are already in prison, said the official, who noted that investigators believe that at least one of the suspects may have converted to Islam while incarcerated.

"From the evidence so far, there is reason to believe that they were planning attacks and that some of those locations may have been targets," said the U.S. official, who asked for anonymity because the investigation continues.

The official said investigators are looking into whether the suspects have links to a radical Islamic group known as Jamiyyat Ul Islam Is Saheeh -- roughly translated as the Assembly of Authentic Islam -- which has been known to have a presence in a state prison where one of the suspects was recently incarcerated. No connections have been made to al Qaeda or any other foreign terrorist group.

"The real fear here is that this might be a truly homegrown type of case, with suspects organizing on their own to carry off some type of operation here in the United States," the official said.

Details of the investigation's latest turn remained sketchy. While the first two suspects remain in a local jail on state charges, the federal charges for the third suspect, Hammad Riaz Samana -- whose arrest last week was first reported Tuesday by the Los Angeles Times -- remain under seal, law enforcement officials said.

Samana, 21, of Los Angeles, is jailed at a federal detention center in downtown Los Angeles, according to the federal Bureau of Prisons Web site.

Officials would not say what led them to Samana or how he may have been connected to the other men, Gregory Vernon Patterson, 21, and Levar Haney Washington, 25, who were arrested early last month during a police stakeout of a gas station.

Both were charged with nine counts of robbery and one count of attempted robbery.

The local investigation was taken over by the FBI's Joint Terrorism Task Force here after police who searched Washington's home discovered the bulletproof vests and jihadist literature. A source said police also found documents with references to Sept. 11, 2001, and a list of sites that included the ticket counter for El Al Israel Airlines at Los Angeles International Airport.

The Los Angeles Times reported that Patterson had worked at a duty-free shop in the airport's international terminal for several months until earlier this year. He had no criminal record. Washington has served time for assault and robbery at Folsom State Prison, sources said.

Patterson's attorney, Winston McKessom, said his client attended California State University at Northridge and met Washington during the course of his studies of Islam. "You're looking at a good kid, no prior record, former honor student, solidly middle class," McKesson said.

Eggen reported from Washington. Special correspondent Sonya Geis contributed to this report.