Mexico City, November 4 (RHC-EFE) --A police general responsible for overseeing security forces in the northern region of the northeastern Mexican state of Tamaulipas has been murdered, authorities in the neighboring state of Nuevo Leon said Monday.

Gen. Ricardo Cesar Niño Villarreal and a woman were found fatally shot inside a vehicle in the municipality of Vallecillo, a spokesperson in the Nuevo Leon Attorney General's Office told EFE news agency.

Niño Villarreal, who was appointed to the regional security post in May, survived a shooting attack last month thanks to the armor-plating on his vehicle, but the car he was driving in Vallecillo lacked armor.

Investigators are working to determine why the general was traveling without bodyguards and using an unprotected vehicle, Nuevo Leon Attorney General Adrian de la Garza said.

For years Tamaulipas, which borders Texas, has been a battleground between the Gulf and Los Zetas drug cartels and is regularly among the Mexican states with the highest numbers of homicides.

President Enrique Peña Nieto sent additional Federal Police and military personnel to Tamaulipas in May and ordered a thorough vetting of the state and municipal police forces to root out corrupt officers.