Terrorists may use secondary explosive devices to kill and injure emergency personnel responding to an initial attack, the FBI has warned law enforcement agencies throughout the country.

In its weekly bulletin sent late Wednesday, obtained by Fox News, the FBI (search) cautioned police officers and others to carefully watch their surroundings.

"These devices may be hidden in everyday objects such as vehicles, briefcases, flower pots or garbage cans, or can be sequential suicide attacks in the same locations, and are generally detonated less than one hour after initial attack, targeting first responders as well as the general population," the bulletin said.

One high-profile example of a secondary attack, according to one analyst, was the second plane to crash into the World Trade Center on Sept. 11, 2001. Jack Spencer, Heritage Foundation (search) senior policy analyst for defense policy, said rescue crews responding to the strike of the first airplane and spectators at the scene were caught by surprise.

"The problem is that once the initial explosion goes off, many people will believe that’s it, and will respond accordingly," Spencer said. He said it is because of concerns about a secondary attack that law enforcement warns people to run away from an initial event.

The FBI specifically mentioned the May 5 bombing of a police station in Athens (search), during which three timed explosive devices were used. The third bomb exploded almost half an hour after the first two, "in what authorities believe was an attempt to injure police officers and emergency personnel responding to the scene."

A Greek radical group claimed responsibility Thursday for the bombings and warned that some visitors to this summer's Olympic Games, such as business executives and wealthy western tourists, would not be welcome.

Although Greek officials said the aim of the May 5 blasts was to injure policemen, Spencer said that with follow-on attacks, there is usually a wider motive at work as well.

The goal is to "incite more terror. If there's an initial explosion and a second explosion, then we're thinking about a third explosion," Spencer said.

The FBI said it possesses no specific information of any planned terrorist attacks in the United States, but noted that law enforcement agencies should remain alert to the use of these types of secondary devices.

"Public safety personnel responding to the scene of a bombing incident are recommended to conduct an immediate search of areas established as staging areas for the investigation," the FBI said.

"Any suspicious items discovered during the search should be isolated, the staging area relocated and bomb squad personnel requested to investigate the item."

Fox News' Anna Stolley contributed to this report.