WASHINGTON (AP) — Authorities monitored a rush of intelligence leads Tuesday at the largest security operation in presidential inauguration history, including a possible threat from an East Africa radical Islamic terrorist group.

Law enforcement and intelligence officials received information that people associated with a Somalia-based group, al-Shabaab, might try to travel to the U.S. with plans to disrupt the inauguration, according to a joint FBI/Homeland Security bulletin issued Monday night. The information had limited specificity and uncertain credibility, said Homeland Security spokesman Russ Knocke.

U.S. counter-terror officials have grown concerned in recent months about the threat posed by the militant al-Shabaab group and a cell of U.S.-based Somali sympathizers who have traveled to their homeland to “fight alongside Islamic insurgents,” the alert reported.

Authorities stressed that the warning was posted as a precaution as part of the massive effort to monitor intelligence traffic and check out all leads in advance of President-elect Barack Obama’s inauguration. Officials have warned that the inauguration poses an attractive target for terrorists because of the large crowds descending on the nation’s capital and the historic significance of the country swearing in its first black president.

“As always, we remind the public to be both thoughtful and vigilant about their surroundings, and to notify authorities of any suspicious activity,” Knocke said.

A senior law enforcement official familiar with the security effort said the Somali alert had been posted to make sure no effort was spared. The official spoke on condition of anonymity because he was not authorized to speak publicly about security matters. The official said authorities have been monitoring suspicious chatter referring to the inauguration in recent days, but as of late Monday night, they felt comfortable with security preparations.

There was an unprecedented amount of security Tuesday, with thousands of law enforcement officers from 58 federal, state and local agencies working together. Sirens keening, squad cars and utility vehicles swept along downtown streets even before dawn, racing to cordoned checkpoints as crowds gathered.

Knocke said officials consistently monitor all threat information, as they always do.

There has been no change in the terrorist threat level, which remains at yellow — or elevated.

Just before 9 a.m. the U.S. Park Police closed the mall at 14th Street and were directing people to monument grounds due to the crowd size, Park Police spokesman Robert Lachance said.

Associated Press Writer Matt Apuzzo contributed to this story