"The car blew up at around 3:40 local time on May 24. As a result the person in the car died," a source in law enforcement told the agency.

"The investigators are looking into the possibility that it was a terrorist attack, but this is not their main hypothesis," the official said, adding that the blast was caused by explosives.

Initial reports said the blast happened outside the headquarters of the security services, but officials later said it was outside a detention centre run by the security services.

A police spokeswoman told the RIA Novosti news agency that "there were people in the car" but could not confirm the number of casualties.

Interfax said witnesses told the agency there were one or two people in the car and that body parts of the passengers were blown out by the force of the blast.

One witness had a video filmed on a mobile phone showing the headless body of a man lying around 10 metres (11 yards) from the car.

A press secretary for the KNB security services told Interfax, however, that a car simply caught fire outside the entrance to the detention centre.

"A car just caught fire and blew up. Nothing special happened here. Any car can catch fire in any place," said spokesman Kenzhebulat Beknazarov.

Copyright AFP (Agence France-Presse), 2011