Officials: No Imminent Threat to U.S. After Hotel Attack

Officials: Pakistan attack has al Qaeda signature.

Sept. 22, 2008— -- A new bulletin issued by the FBI and the Department of Homeland Security after Saturday's deadly attack at a Pakistan hotel says the agencies have found no evidence or leads that point to an imminent attack against the United States.

Additionally, U.S. officials conclude that, while it's early, the attacks bear the signature style of al Qaeda.

The attack at the Marriott Hotel in Islamabad killed 53 people, including two Americans, and injured more than 250, according to the Pakistani government.

U.S. officials have developed some investigative theories about the bombers' means for carrying out the attack.

They believe the huge bomb was able to leave a crater 30-feet deep because of its construction: a six-wheel truck packed with about 2,000 pounds of explosives, grenades and mortars, and laced with aluminum powder to speed the explosion and increase the intensity of the resulting inferno.

Additionally, U.S. officials believe terrorists impersonated government security personnel and used small arms to get close enough to detonate their massive bomb, and also appear to have taken advantage of weaker security because of changes in work habits during the holy month of Ramadan.

U.S. officials also think it's likely that the terrorists sought to assassinate newly-elected Pakistani President Asif Ali Zardari. Investigators believe accounts that Zardari and other Pakistani leaders were scheduled to dine at the targeted hotel, but changed their venue at the last minute.

Officials also expect more attacks, because some Islamic radicals maintain martyrs receive additional heavenly benefits during Ramadan.

Since Saturday's hotel attack in Islamabad, several other violent incidents have occurred, including the kidnapping of the incoming Afghanistan ambassador, and a suicide bomber who killed nine Pakistan army troops in Swat.

ABC News' Nick Schifrin contributed to this report.