A federal agent was fatally shot outside a busy south Florida post office after a fight.

The U.S. Customs and Border Protection agent died in hospital after the 9 a.m. shooting.

Donald Pettit, 52, was with his young daughter when he was shot, said Carlos Baixauli, a spokesman for the Bureau of Alcohol, Tobacco, Firearms and Explosives.

Pettit was shot once by a man during a possible fight in the post office parking lot, officials said. No one else was injured, and it wasn't clear whether the shooting was related to the agent's job. The suspect drove off in a car and police were combing the area for him.

A government helicopter hovered overhead and police officers appeared at every major intersection near the shooting scene.

Police later issued a sketch of the suspect based on witness descriptions: A white man, possibly Hispanic, between 50 and 60 years old. They said the man was about six feet tall, medium to heavy build, with grey and white bushy hair and "distinctive sagging cheeks."

Investigators interviewed potential witnesses while oking for any surveillance cameras that might have recorded the shooting.