They said some evidence was found indicating the al Qaeda leader or his associates had engaged in discussions or planning for a possible attack on a train inside the United States on Sept. 11, 2011.

"We have no information of any imminent terrorist threat to the U.S. rail sector, but wanted to make our partners aware of the alleged plotting," spokesman Matthew Chandler said of an intelligence message the Department of Homeland Security sent on Thursday.

The department and other U.S. agencies have been reviewing the treasure trove of information from bin Laden's compound in Pakistan seized by the United States during the raid this week that killed the al Qaeda leader.

An initial review of the information by U.S. intelligence analysts indicates that bin Laden, while in Abbottabad, played a direct role for years in plotting terror attacks, and was not just an inspirational figure to al Qaeda, The New York Times reported on Thursday.

"He wasn't just a figurehead," the Times quoted a U.S. official as saying. "He continued to plot and plan, to come up with ideas about targets, and to communicate those ideas to other senior Qaeda leaders."

The information on plotting against the U.S. rail sector indicated one possible tactic for attacking a train was trying to tip it somehow off its tracks, one official said.

The official said it appeared from the information that this was an idea that bin Laden or his associates considered, but there was no indication now from the intelligence that further plans were drawn up for the scheme or that steps were taken to carry it out.

Another official said al Qaeda in February last year contemplated the rail attack

to occur on the 10th anniversary of the hijacked plane attacks on the World Trade Center in New York and the Pentagon, but the group was not tied to that exact date.

Since the raid, the Department of Homeland Security has taken a number of steps in reviewing measures at all potential terrorist targets, including transportation systems across the country. It added more officers at airports and at the borders.

Chandler said the alleged al Qaeda plot was based on "initial reporting, which is often misleading or inaccurate and subject to change." 

He added, "We remain at a heightened state of vigilance," but said there were no plans to raise the national threat level.

Officials have long been concerned that al Qaeda might try to carry out attacks on the U.S. rail system.

In 2008, U.S. authorities warned of a possible al Qaeda threat to transit systems in and around New York City over the Thanksgiving holiday weekend.

Last year, an Afghan immigrant pleaded guilty in New York to plotting a suicide bombing campaign on Manhattan's subway system in what U.S. authorities described as one of the most serious threats since the Sept. 11 attacks.

( Source: Reuters )