PEMBROKE PINES, Fla. -- A $275,000 reward is now being offered for information leading to an arrest in the murder of a U.S. Customs and Border Protection agent outside a Pembroke Pines post office Tuesday morning.

Authorities also released a sketch of the man they said shot and killed the agent.

Donald Pettit, 52, was fatally wounded during an argument outside the South Florida Mail Processing Center, located near Dykes Road and Pines Boulevard.

According to police, Pettit and the gunman got into an argument in the parking lot. That's when the gunman fired a shot, striking Pettit in the head.

Pettit was taken to Memorial Regional Hospital, where he later died. The gunman fled the scene.

A motive for the shooting was not immediately known, but police said Pettit was not armed.

"He did have his child in the car with him," Pembroke Pines police Deputy Chief Mike Segarra said, adding that it made the shooting more "sickening."

Pettit, who lived in Pembroke Pines with his wife and two children, served in the U.S. Army for 20 years and spent 15 years as an investigator with the Food and Drug Administration before joining U.S. Customs and Border Protection.

Police said the shooter is believed to be in his 50s or 60s, 6 feet 1 inch tall with white, slicked-back hair, saggy cheeks and a medium build. Police said he was driving a green or gray Chrysler 300.

A command post was established in the parking lot of the post office. Federal agents, state law enforcement, police officers and Broward Sheriff's Office deputies have joined in the search for the shooter and the vehicle.

"We're here to work together and we're going to find this guy," Segarra said.

Anyone with information regarding this incident is urged to contact the Pembroke Pines Police Department Detective Bureau at 954-431-2225, E-mail Tips@ppines.com, or call Crimestoppers at 954-493-TIPS.