Polish investigators have charged two people with murder in the 1998 killing of the country's former national police chief, Marek Papala.

The head of the national police investigating team, Marek Dyjasz, said testimony from a suspect arrested last year indicates that Papala was killed during an attempted robbery of his car.

He said Wednesday that four people were arrested Monday in the case. Two were charged with murder and armed robbery, and three others _ including the one arrested last year _ were charged with armed robbery.

Papala was found shot in the head in his car in front of his Warsaw apartment block in June 1998. A few months earlier a right-wing government had fired him. The case shocked the nation and fueled theories of gang or political revenge on Papala, who was linked to ex-communists.

Earlier testimony from another suspect led Poland to ask the United States in 2007 to extradite Edward Mazur, a Chicago-based millionaire businessman of Polish origin, on suspicion that he had offered money to have Papala killed.

Mazur was put in custody but a U.S. court rejected Poland's extradition request.