WASHINGTON – Authorities monitored a rush of intelligence leads yesterday 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 operations 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 early yesterday, they felt comfortable with security preparations.

There was an unprecedented amount of security yesterday, 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.

Law enforcement responded to several suspicious packages and vehicles yesterday morning, FBI spokesman Richard Kolko said. Many of the packages were found in trash bins near check-in points. People who were prohibited from carrying certain items onto the Capitol grounds had to throw the items away to enter.

After the swearing-in, spectators tore down some fencing along the mall instead of exiting through designated areas. No one was injured, according to the Park Police, and the fencing was restored.

Around 1:30 p.m. there was a bottleneck at 14th Street, a major thoroughfare, where people were trying to find a place to get off the mall.

In addition, subway service was disrupted on one of Washington’s main Metro arteries after a woman fell on the tracks at a downtown station. She was hospitalized with non-life-threatening injuries. The Metro subway system was running at crush capacity since 4 a.m.

D.C. fire and EMS department spokesman Alan Etter said medical personnel were having trouble getting to people quickly around the National Mall because of the throngs of people, but that everyone who has needed help has eventually received treatment.

“Obviously the crush of people downtown is making it very challenging,” Etter said. “We’re doing the best we can.”

Between 4 a.m. until 10 a.m., the fire department responded to more than 60 calls from people falling down or complaining of the being cold, Etter said. About 20 people have been hospitalized.

Two of those hospitalized were teenage girls with breathing problems who came from the First Aid tent at the Natural History Museum. A volunteer at the aid station said subfreezing weather and the long wait had begun taking its toll.

Two soldiers helped a woman into the tent where she was also treated for breathing problems, then was allowed to lie on a cot. About 10 others had come in for warmth and for a chance to lie down. The volunteer, who was not authorized to speak publicly, said dozens had come for footwarmers and handwarmers.

And just after 10:15 a.m., police stationed near the Agriculture Department began diverting the gridlocked crowds toward the Washington Monument because that section of the mall was inundated with spectators. Frustrated visitors at one point began clambering over concrete barriers, ignoring outmanned police, until a group of military security officials arrived and secured the area.