The slaying of a 20-year-old man remains a mystery more than a month after he was found fatally shot in the cab of his idling pickup parked on street in northwest Houston.

The body of Richard "Richie" Paul Corbello Jr. was discovered about 7:20 a.m. Dec. 19 slumped behind the steering wheel of a black Chevrolet Silverado pickup in the 5300 block of Lost Forest near Pinemont and not far from Clifton Middle School, according to the Houston Police Department.

HPD Officer M.L. Stahlin said a resident spotted the man in the truck. The engine was idling. The resident tried to get the man's attention but couldn't and then discovered the man had been shot. The resident called 911.

Officials said Corbello had been shot once in the back of the head. No gun was found at the scene.

Homicide investigators so far have been unable to determine a motive for the killing or who pulled the trigger.

Two men who spotted the body said they had not seen and heard anything to indicate gunfire before the discovery.

Michael Vargas said he saw the truck parked at the curb and the engine idling when he came to pickup his friend to go to work. He said he looked inside and thought the man was passed out behind the wheel.

Vargas said he went to his friend's home and told him about the man. Then they both went outside to investigate. They knocked on the door and the hood but the man didn't respond. Then they opened the door.

"When we opened the door we saw a guy who had been shot in the head," said Roland Mendiola. "All the blood was coming out and blood was splattered on the seat."

Mendiola and other residents said they had never seen the truck in the neighborhood before.

"I hope they find out what happened to the guy," Mendiola said. "Nobody deserves to get shot like that in the head."

Anyone with information about the case is urged to contact identity Crime Stoppers of Houston at 713 222-TIPS or on-line at Tips can also be sent by text message. Text TIP610 and tips to CRIMES (274637).

Crime Stoppers will pay up to $5,000 for any information that leads to the arrest of a suspect or the filing of felony charges in case.

All tipsters remain anonymous.