MEXICO CITY – Gunmen killed nine people at an auto repair shop in the gang-plagued city of Culiacan, while a police investigator was found shot to death near the city's police headquarters, prosecutors said Thursday.

Culiacan is the capital of northern Sinaloa state – home to the powerful Sinaloa drug cartel and site of an ongoing wave of drug-related violence.

Six bullet-ridden bodies were found Thursday inside the auto body shop, and three more bodies were found on the street just outside the business, the state prosecutor's office said in a statement.

None of the nine bodies have been identified and no arrests have been announced.

A police investigator was found shot to death in his truck near Culiacan's police headquarters. It was unclear if the same gunmen were involved in his death.

On Wednesday, ten police officers in the town of Agua Prieta, across the border from Douglas, Arizona, resigned after a fellow patrolman was gunned down.

Mayor Antonio Cuadras said Thursday that authorities tried to get the police to stay by offering them better weapons and bulletproof vests.

Mexico has had trouble keeping police as violent drug gangs increasingly target officers.

U.S. officials say at least three Mexican police commanders have crossed into the United States to request asylum because they fear for their lives.