The federal government is warning local authorities of possible attacks on the U.S. transportation system this summer, U.S. officials told Fox News.
The FBI (search) and Department of Homeland Security (search) sent out warnings Thursday night saying there is uncorroborated information pointing to possible attacks on buses and trains, specifically, near major U.S. cities.
"The plot calls for the use of improvised explosive devices possibly constructed of ammonium nitrate (fertilizer) and diesel fuel concealed in luggage and carry-on bags to include duffel bags and backpacks," states the warning, titled, "Subject: Alleged Plot Against Major U.S. Cities in the Summer of 2004." Ammonium nitrate (search) is the chemical that was used in the Oklahoma City bombing.
The two agencies issued the warnings to law enforcement and security personnel throughout the country.
The warning noted that Al Qaeda and other groups have proved they can attack public transportation with conventional explosives, vehicle-borne bombs and suicide bombers in places such as Greece, Israel and Turkey. The recent railway attacks in Madrid used plastic explosives concealed in backpacks.
During terror raids in London this week, officials found and seized 1,000 pounds of ammonium nitrate. A group of eight British citizens of Pakistani origins were arrested on suspicion of being involved in possible terrorist attacks.
The possibility of these attacks has been a factor as officials contemplate how to best secure locations in Boston and New York City this summer, the sites of the Democratic national conventions and the Republican conventions, respectively.
One option New York City is considering, for example, is to shut down Penn Station — which is below Madison Square Garden (search), the site of some GOP convention activities — during major addresses, such as when President Bush is on stage or when he is in the building.
But security experts said shutting down buildings or areas like Penn Station (search) isn't realistic.
"That's really impossible — they'd really be shutting down the economy," former NYPD squad commander Joe Cardinale told Fox News. "You're going to have to just step up all the efforts" and coordinate with local law enforcement, rail workers, unions and others on security, he added.
Thursday's warning was not part of the weekly bulletin usually sent out on Wednesdays, which this week covered visas, but a special, non-scheduled warning.
In Thursday's warning, the FBI and DHS issued several guidelines for state homeland security advisors and staff, Department of Transportation, state emergency managers, the Transportation Security Administration and other law enforcement and security personnel.
— Ensure all access portals have appropriate security measures, including surveillance.
— Approach all illegally parked vehicles in and around facilities.
— Monitor terminal parking lots for suspicious loitering.
— Remove trash receptacles.
— Ensure terminals and facility perimeters have sufficient lighting.
— Arrange for law enforcement to be parked near entrances and exits.
— Deploy an overt security presence outside public transpiration entrances.
The FBI has previously sent out warnings about threats to subways and railways. For example, On Jan. 28, the FBI said, "recent intelligence indicates a continued terrorist interest in conducting attacks on U.S. subways and railways." Furthermore, the March 11 bombing in Madrid has increased law enforcement concern about possible activity in the United States.
Since then, much a lot of law enforcement attention in recent weeks has focused on Amtrak and local railroads. Railroad security is lagging compared to security at U.S. airports; baggage, for the most part, isn't searched, passengers don't go through metal detectors and a train can often be boarded without even presenting a ticket or identification.
DHS recently announced initiatives to increase rail security, including bomb-sniffing dogs to be made available to local law enforcement and a pilot program to test luggage that's being brought aboard trains.
One official told Fox News that since the Madrid bombings, there has been a certain amount of "chatter," "including boasting and bragging." The chatter indicated a U.S. transportation plot.
Other officials note that the information, however, is "uncorroborated, non-specific threat reporting."
A DHS official told Fox News that there are no plans to raise the national threat level based on "a couple" of uncorroborated threats.
"We overheard people talking about Madrid, then saying you should see what's going to happen later on," the official said of one conversation picked up in recent chatter.
Given the severity of the Madrid attack and the recent arrests in London, which one official believes probably thwarted a terrorist attack, it was deemed wise to issue the bulletin, the official said.
One DHS official said once a possible threat like this is deemed worthy of noting, law enforcement requests that those areas threatened take proper action. It is up to them — states, local officials, private sector — to decide how they will act.
Some of those actions may include increased police presence, announcements to passengers advising them to keep their eyes open for random bags and other acts.
Fox News' Mike Emanuel, Catherine Herridge and Anna Stolley contributed to this report.