Police in Mexico are investigating the killing on Friday of controversial General Mario Acosta Chaparro.
Gen Acosta was shot dead by an unidentified gunman as he dropped off his car at a garage in Mexico City.
The retired general had been convicted of protecting one of Mexico's most powerful drug lords, but was later cleared of the charges.
Gen Acosta had also been investigated for human rights abuses, but the charges were dismissed.
He had survived a similar attack in 2010.
Police said Gen Acosta was shot three times in the head by the gunman, who approached the General at a garage in the Anahuac neighbourhood of Mexico City.
He is one of the highest-ranking military officials to be killed in Mexico in recent years.
In 2000, Gen Acosta was arrested on suspicion of protecting Amado Carillo Fuentes, a leader of the Juarez drug cartel.
He was found guilty and sentenced to 16 years in prison.
But in 2007, an appeals court said prosecutors had failed to prove his alleged links to Amado Carillo Fuentes.
He was freed and given back his rank of general.
He was also accused of participating in the disappearance of leftist activists in the 1970s and 80s, but the charges against him were dismissed.
Police said the fact that Gen Acosta was shot in the head three times from a short distance suggested the motive was not robbery, but a settling of scores.
Gen Acosta had survived an earlier attack on him two years ago.
On that occasion, he was shot in the stomach. The assailants were not caught.