MEXICO CITY (AP) — The police chief of the violent Mexican border city of Nuevo Laredo is missing, authorities said Monday.

Tamaulipas state prosecutors said they have opened an investigation into the whereabouts of Roberto Balmori Garza, police chief of the city across the border from Laredo, Texas.

Tamaulipas prosecutors said in a brief statement that state officials in Nuevo Laredo will be in charge of the investigation.

Local media reported that two of Balmori Garza's brothers were found shot dead Sunday inside the trunk of a car in the neighboring state of Nuevo Leon. One of his brothers was a federal investigator, media reported.

Tamaulipas prosecutors' spokesman Ruben Dario said Balmori Garza disappeared over the weekend. He said he couldn't give any other information on the case or confirm the media reports of the death of Balmori Garza's brothers.

Nuevo Laredo, a stronghold of the Zetas drug cartel, has been the scene of bloody drug-gang turf battles since the beginning of the year.

Two years ago, gunmen killed a retired army general who had been police chief of Nuevo Laredo for a month. Two of his bodyguards also were slain and two suffered wounds.

The Zetas, known for its viciousness, has been fighting its former ally, the Gulf cartel, in Mexico's northeast since early 2010.