At least some of the recent police murders in the Mexican state of Sonora were carried out by people with military training, the state's governor said on Thursday.

Gov. Eduardo Bours was quoted by the Hermosillo-based newspaper El Imparcial as saying that Sonoran officials had "substantial" information on new hit squad or squads now operating in the state.

"These are military-trained people contracted to do what they came to do," Bours said.

He would not elaborate further.

The Sonora state police reportedly have three people in custody who are suspects in police killings, and one has been identified as Manuel Herrera Villalba, a former member of the special operations amphibious unit of the Mexican army.

Herrera Villalba is suspected in the killing of a state police officer in Hermosillo, the capital of Sonora state. The officer was shot Monday night in the parking lot of the Sonora Attorney General's Office.

El Imparcial is also reporting that a man arrested on Thursday in connection with an unspecified police murder was a Mexican army deserter.

Sonora, which had seen a 14 percent decline in murders in 2006, has suffered a recent rash of drug-related violence. More than 20 killings connected to organized crime have been registered in the state so far in 2007, a number of recent attacks have involved police officers.

On Feb. 26, the head of public safety in Agua Prieta, Ramon Tacho Verdugo, was shot and killed by a group of unknown assailants as he left his office. Two days later, a member of the federal preventative police force was shot and killed in the city of Magdalena de Kino.

On Tuesday, a city cop in Cananea was shot dead in his patrol car, and that same day, the tortured body of an Hermosillo municipal policeman was found outside the city.

Officials and analysts in the U.S. say the recent surge in violence is a result of new criminal elements moving into Sonora to challenge a group of loosely affiliated cartels known as "The Federation," which controls the state's lucrative smuggling routes.