Washington (CNN) -- With the architect of the September 11, 2001, attacks dead, U.S. commandos in Osama bin Laden's Pakistan compound quickly came upon another prize: A trove of hard drives, DVDs and thumb drives that might provide evidence of other al Qaeda operations.
Those materials, seized after bin Laden and four others were killed, have provided the first specific alert to come out of Monday's raid.
The U.S. Department of Homeland Security released a notice tied to rail security Thursday.
The unclassified notice to "federal, state, local and tribal partners" says that, in February 2010, al Qaeda members discussed a plan to derail trains in the United States by placing obstructions on tracks, according to a law enforcement source who received the notice.
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The plan was to be executed this fall to coincide with the 10th anniversary of the September 11 attacks. But no specific city or rail system was identified in the notice, the source told CNN.
Although a U.S. official cautioned that the information doesn't appear to suggest an imminent plot against U.S rail systems, the discovery may be the tip of the iceberg of potential plots.
The haul included "lots" of paper documents, 10 hard drives, five computers and more than 100 storage devices, such as disks, DVDs and thumb drives, a senior U.S. official said Wednesday.
Since Monday, intelligence experts and others have pored over the find.
A task force was set up at the CIA to comb through the evidence seized in the raid, according to a senior intelligence official who briefed reporters on condition of anonymity.
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At a Senate Judiciary Committee hearing Wednesday, U.S. Attorney General Eric Holder said of evidence taken from the residence, "As we glean information from that material, we will make appropriate decisions with regard to who might we add to the terrorist watch list, the no-fly list, all those things."
The U.S. official who spoke Thursday of the data said, "We take every possible threat seriously, so we are running all of this to ground."
When asked if there were any indications that bin Laden had signed off on the rail operation, the official said, "There is no information to suggest that there was some kind of 'blessed operation' attached to U.S transportation systems."
The same U.S. official says the al Qaeda scheme, which included derailing trains over bridges and valleys, "doesn't appear to be anything more than an idea on paper at this point."
Material also indicated that al Qaeda was particularly interested in targeting cities -- Washington, New York, Los Angeles and Chicago, the official said. Authorities sifting through the material learned al Qaeda has considered striking on significant dates such as July 4, Christmas and the opening day of the United Nations, according to the official.
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Department of Homeland Security spokesman Matt Chandler echoed the belief that there was no imminent threat to the rail system. But officials "wanted to make our partners aware of the alleged plotting," he said. "We want to stress that this alleged al Qaeda plotting is based on initial reporting, which is often misleading or inaccurate and subject to change."
Since Sunday, Chandler said, the department and other agencies have reviewed potential terror targets and deployed additional officers to nonsecured areas at U.S. airports.
Homeland Security Secretary Janet Napolitano told federal lawmakers Wednesday a "number of actions" have been taken in response to the killing of bin Laden, among them "surging some resources" to U.S. ports, airports and borders.
"These include issuing advisories to fusion center directors, Homeland Security advisers, major city chief intelligence commanders, private sector critical infrastructure owners and operators and other law enforcement entities," she said at a Senate hearing.
"We are and have been reviewing all open cases of potential al Qaeda core, AQAP (al Qaeda in the Arabian Peninsula) and AQIM (al Qaeda in the Islamic Maghreb) operatives possibly in the U.S. in conjunction with the FBI," she added.
Sen. Joe Lieberman, the Connecticut independent who is chairman of the Homeland Security and Governmental Affairs Committee, asked Napolitano about the data removed by the Navy SEALs from the bin Laden compound. "I assume that as this material is gone over, anything related to Homeland Security will be shared immediately with your department?" he asked.
Napolitano responded, "It is being shared (already)."
Earlier this week, former CIA Director Michael Hayden told CNN's "The Situation Room" that the "diffuse" nature of al Qaeda is among its strengths.
"Keep in mind this is a network, not a hierarchy," Hayden said. "So you've got still very active, very talented cells around the world."
CNN Homeland Security Correspondent Jeanne Meserve and Jim Barnett contributed to this report.