The next domestic terrorist attack may come in the form of homicide bombings, the FBI warned law enforcement officials in a bulletin released on Thursday.

In the lightly classified intelligence bulletin, the FBI (search) said some telltale signs of a homicide bomber would be heavy bulky clothing on warm days, a chemical smell or people wearing jackets with wires hanging out. Bombers may also tightly clench their fists in which they could be gripping detonators.

The bulletin also warns that a terrorist could disguise himself by wearing a public safety uniform or by dressing as a pregnant woman.

"The most important thing we have is people's awareness," said Richard Dietl, a former New York Police Department detective. "You've got to go out there and tell the police if you think something is wrong."

Officials said the bulletin was not prompted by new information but served as a reminder for local law enforcement to be on the lookout. The Department of Homeland Security (search) has been on alert for such attacks for some time. In March 2003, Homeland Security Secretary Tom Ridge told Fox News he believed "we have to prepare for the inevitability" of homicide bombings in the United States.

The bulletin, which was issued to 18,000 law enforcement agencies nationwide, comes as there are increased fears about a variety of threats.

The discovery last week of a concealed infrared electronic device on train tracks near Philadelphia sparked investigations and renewed concern for rail safety in the northeast corridor.

"The conclusion has not been reached as to why it was there," said Asa Hutchinson, the Homeland Security under secretary for border and transportation security. "It's unexplained. It shouldn't have been there."

Rail safety has been a heightened concern since the March Madrid train bombings (search), which killed nearly 200 people. Some law enforcement personnel worried that attack could be a prelude to a similar bombing in America.

Hutchinson told Fox News that in light of the Madrid attack, "we're being more careful." Commenting on rail security, he added, "A lot is being done that the public is not aware of, from surveillance and sensors to uniformed police."

But lawmakers are unconvinced, butting heads with the administration and complaining that rail security efforts are moving too slowly.

In a congressional hearing earlier this month, Rep. Ed Markey (search), D-Mass., railed against the administration's efforts to improve security. "Although we have seen what happened in Madrid, we have deployed just a fraction of the resources possible," Markey said.

On Thursday, the Department of Homeland Security announced new measures to expand rail security. The directives include instructing rail owners and operators to ask passengers and employees to report unattended property or suspicious behavior.

It also requires operators to use bomb-sniffing dogs when needed and in some locations to remove trash receptacles, except clear plastic or bomb-resistant trash containers.

These measures are part of a series of initiatives that include vulnerability assessments, more training for rail personnel and more funds to improve rail and transit security in urban areas.

A sign of the added security and worry is that two of Amtrak's Washington bound Acela high speed trains were stopped Thursday night and searched by Amtrak police officers and canines. The trains were delayed for 40 minutes, but there is no word yet specifically on why they were stopped.

Trucks coming into New York City are also being scrutinized as there is a continuing concern about high profile targets there.

Hutchinson said defending the public from a truck bomb is particularly difficult. "In a free society, you can't stop every truck," he said, adding it is important for the industry to be proactive in policing itself and identifying threats.

Law enforcement officials tell Fox News that much of the information out there is unspecific, but overall the picture points to foreign terrorists planning a terror strike in the United States.

Fox News' Greg Kelly contributed to this story.