HOUSTON -- A man accused of shooting and killing a Houston police officer during a traffic stop said he is not guilty by reason of insanity, KPRC Local 2 reported Monday.

Juan Leonardo Quintero, 33, is accused of fatally shooting veteran Houston police Officer Rodney Johnson in September 2006.

Quintero has been charged with capital murder of a police officer.

Quintero offered to plead guilty in exchange for a life sentence without parole, but prosecutors rejected the deal.

Prosecutors told jurors in opening statements about Johnson's loving family.

"He was a father of three -- three children who loved him very much. He was a husband to another police officer, who adored him. He was a stepfather to two people who cherished him," Harris County assistant district attorney John Jordan said. "The defendant was not insane. He was just a cold-blooded killer."

Houston police said Quintero shot Johnson four times in the head after Quintero was put in the back of a patrol car during a traffic stop in southeast Houston. Investigators said Quintero used a 9 mm handgun he had hidden in his waistband.

A tow-truck driver who was first to arrive at the scene testified that Quintero almost killed him, as well. He said that when he leaned over Johnson's body to radio for help, Quintero fired at him.

"The bullet grazes over his head within inches of killing a second person in five minutes," Jordan said.

Investigators said the first officers at the scene found Quintero handcuffed in the back seat of Johnson's patrol car, with a gun in his hand.

Quintero was pulled over for speeding and driving without a license. Quintero's attorney said a childhood injury left him with brain damage.

"His ability to understand social situations and social cues, and respond to people and understand and filter that is all very damaged. So, he can't read the situation the way you and I can," defense attorney Danalynn Rercer said.

The defense said that Quintero thought his life was in danger when he was in the patrol car.

Johnson's family and fellow officers were in the courtroom Monday.

Police Chief Harold Hurtt testified that Johnson was an exceptional officer.

Quintero's defense attorney said uniformed officers in the courtroom could hurt his client's chances of receiving a fair trial. The judge denied the motion and ruled that the officers could remain in the courtroom.

Quintero is a Mexican citizen who was deported in 1999 after being convicted of indecency with a child, but sneaked back into the country, according to authorities.