The modified cell phones turned up during searches in Saudi Arabia following the May 12 bombing in Riyadh that killed 35 people, including nine Americans, according to a weekly FBI bulletin to 18,000 state and local law enforcement agencies.
Although the FBI said it has no information indicating cell phones would be used by terrorists in the United States, the bulletin urged local officials to take several precautions if a suspected device is found.
For instance, officers should "immediately evacuate the area to a minimum distance of 300 yards. Radios, cellular telephones and pagers should not be used within 50 feet of the suspected device," the bulletin said.
Terrorists also have used pagers and radio systems to detonate bombs by remote control, the FBI said.
The bulletin includes details of how a cell phone can become part of a deadly bomb. It includes use of a battery, a switch, an initiation device such as a match or a light bulb, conducting wires and explosives.
When the phone receives an incoming call, "the electrical power from the telephone's ringer or vibrator activates the bomb's circuitry" causing an explosion.
"Law enforcement officers without specialized explosives training should never attempt to remove or disable a suspected device," the bulletin warned.
The bulletin did not say whether cell phones were used in the May 12 bombings, nor were there other details about the searches that uncovered the suspicious phones. Saudi officials said last week they had identified 12 of the attackers and had 25 people in custody in the case.