Federal officials are warning law enforcement agencies in the United States that terrorists might strike trains and buses in major U.S. cities in coming months with explosives hidden in bags or luggage.

A Federal Bureau of Investigation bulletin says al-Qaida and other terrorist groups have demonstrated the intent and capability of attacking public transportation systems using a variety of bombs. The bulletin notes last month's train bombings in Madrid that killed more than 190 people.

Meanwhile, the U.S. government says a program to photograph and fingerprint foreign visitors will be expanded beginning September 30 to include travelers from some of America's closest allies.

The Homeland Security Department says the change affects citizens from 27 countries allowed to travel without a visa in the United States for up to 90 days. The list of countries includes Britain, Japan, Australia and New Zealand.

Visitors from other countries already face those procedures, which were implemented after the September 11 terrorist attacks.

In another bulletin sent to law enforcement agencies on Wednesday, the FBI said terrorists may attempt to enter the United States using cultural, arts or sports visas, known as P-Visas. It says recent intelligence indicates that terrorist groups may want to exploit the State Department's P-Visa program to place operatives on U.S. soil.