The warning cited uncorroborated intelligence as pointing to a bomb made out of ammonium nitrate fertilizer and diesel fuel, similar to the one used to blow up the Oklahoma City federal building in April 1995.
A “viable” explosive constructed of ammonium nitrate and diesel “could be concealed in standard luggage,” the bulletin warned.
Authorities said recent intelligence, paired with the well-coordinated train bombings in Madrid, has increased the level of concern that terrorists are planning an attack in the United States.
Al-Qaida and other terrorist groups have “demonstrated the intent and capability” to attack public transportation with a variety of bombs, including suicide bombers, the bulletin says, according to the Associated Press.
Included in the bulletin were suggestions on how to increase security, such as close monitoring of parking lots, removal of trash receptacles, limiting access points, improving lighting and beefing up overall law enforcement presence.
Authorities suggested checking delivery vehicles parked near public transportation exits and entrances and random security sweeps of passengers, in addition to the familiar public announcements asking people to report suspicious activity.
The warning comes a day after an FBI bulletin to state and local law enforcement agencies said terrorists might try to use cultural, artistic or athletic visas to slip into the United States undetected.