The FBI's counterterrorism unit has launched a broad investigation of US-based theft rings after discovering some vehicles used in deadly car bombings in Iraq, including attacks that killed US troops and Iraqi civilians, were probably stolen in the United States, according to senior US Government officials.

The FBI's deputy assistant director for counterterrorism, Inspector John Lewis, said the investigation did not prove the vehicles were stolen specifically for car bombings in the Middle East, but there was evidence they were smuggled out of the US by organised criminal networks that included terrorists and insurgents.

Cracking the car-theft rings and tracing the cars could help identify insurgent leaders and shut down one of the means used to attack the US-led coalition and the Iraqi Government, the officials said.