U.S. President Barack Obama praised the skill and courage of the American military and intelligence agencies following the killing of the world's most wanted fugitive.
Obama said that al Qaeda mastermind Osama bin Laden "will never threaten America again" during a speech at Fort Campbell, Ky., Friday, where many troops have recently returned from Afghanistan.
Earlier, a vengeful al Qaeda publicly acknowledged bin Laden's death, while warning of retaliation against the country that hunted down and killed the notorious terror leader.
On Friday, al Qaeda released a statement through militant websites pledging to remain "a curse chasing the Americans and their agents, following them outside and inside their countries," in the wake of bin Laden's death.
The message signed by "the general leadership" of al Qaeda could not be independently confirmed, though it was posted on websites where the group usually puts out its messages.
The confirmation came less than a week after Obama announced that bin Laden had been killed in Pakistan by U.S. forces.
The U.S. had been tracking bin Laden ever since the devastating attacks of Sept. 11, 2001, but had been unable to catch the man until the CIA tracked him to a fortified compound north of Islamabad.
U.S. intelligence watched the property for months and Obama gave the authorization to strike last weekend, leading to the commando raid in which bin Laden was killed.
While bin Laden's death has been met with jubilation by many Americans, it has also sparked fears of a new round of terror attacks -- the type that al Qaeda warned would see happiness "turn to sadness" in the U.S.
Following the raid on bin Laden's compound, the U.S. rounded up various computers, DVDs and documents that were found where he was slain.
U.S. officials quickly began reviewing the materials, in which they found bin Laden's apparent plans to launch a rail attack in the United States at a future date.
A set of handwritten notes described a plan to tamper with a railway track in a manner that would cause a train to fall off a valley or bridge. U.S. officials believe bin Laden was working on the plot as recently as February 2010, though it was still in the planning stages.
The FBI and Department of Homeland Security released an intelligence bulletin to American law enforcement agencies around the country, alerting them to the details of the unfinished rail attack plan.
A U.S. official told The Associated Press that other documents revealed bin Laden's desire to strike targets in major American cities, on holidays and anniversaries -- such as the 10th anniversary of the Sept. 11 attacks.
It warned for law enforcement to be on alert for any indications that a particular train or section of track could be vulnerable, but conceded the government had "no information on possible locations or specific targets."
Matt Chandler, a Homeland Security spokesperson, said the details of the "alleged al Qaeda plotting is based on initial reporting, which is often misleading or inaccurate and subject to change."
U.S. Senator Susan Collins has said Homeland Security Secretary Janet Napolitano should raise the country's threat level while the confiscated bin Laden materials are still being reviewed.
"I continue to question the secretary's decision not to increase the threat level," said Collins, the top Republican on the Senate Homeland Security and Government Affairs Committee.
Chandler said the U.S. government has no plans to issue an official terror alert because of the discovery of the plot that bin Laden was apparently working on.
With files from The Associated Press
