Federal officials are investigating a violent Central America-based street gang for ties to Middle Eastern terrorism, an alliance that seems unlikely but poses a frightening threat to the United States.

MS-13, also known as La Mara Salvatrucha (search), has about 15,000 members in the United States in cities from Los Angeles to Boston. Some members have been charged with crimes that include rape, carjacking and drug smuggling.

Officials suspect MS-13 makes money by shuttling illegal aliens across the Mexican border into the United States. According to local police, several Middle Eastern men have been arrested at the border with MS-13 members during the past year, leading some to believe that terrorists are piggybacking on South American illegals to enter the United States.

"We've had recent information that they have been involved with not only narco-terrorists but Al Qaeda and that type of thing," said Hunter Glass of the Fayetteville, N.C., police force.

The evidence so far is mostly circumstantial, but there are indications Al Qaeda (search) has approached MS-13 for help crossing the border.

"They were trying to make a deal with people like us, gang members, from La Mara Salvatrucha, 'cause they know we transport people," said one MS-13 member. "They asked me if Muslim people were to pay me this ransom money, would I transport them over here?"

But federal law enforcement agents say they don't believe there is a link between MS-13 and Al Qaeda. The Department of Homeland Security (search) said it cannot find any evidence of a connection.

"We've heard many of those reports over the summer and through the autumn, and we have not been able to corroborate that, in fact, MS-13, to be specific, has been meeting with Al Qaeda," Homeland Security official John Torres told FOX News.

The FBI said it's implausible that radical Muslims would align with a predominantly Catholic criminal gang.

But two Texas congressmen have written a letter to President Bush warning that 15,000 non-Mexican illegal aliens have crossed the border in recent months, many of them of Middle Eastern origin. The Department of Homeland Security said it is looking into the matter.

Click in the box near the top of the story to watch a report by FOX News' Alisyn Camerota.