OAKLAND -- A 44-year-old Pacifica man, injured by glass fragments when a gunman opened fire on his SUV Sunday morning on an Oakland freeway, doesn't know why he was targeted, police said Monday.

The man's name was not released. He was treated and released Sunday for his injuries, which were mainly to his head, police said. Three passengers in the man's 2005 maroon Cadillac Escalade were not hurt.

The man told investigators he did not know why anyone would want to shoot at him. Police are trying to determine if the shooting involved road rage or something else,

The shooting happened about 2:40 a.m. Sunday in the westbound lanes of I-580 just east of the I-80 turnoff.

Police said the man and his passengers were headed to San Francisco after leaving a downtown Oakland nightclub.

The man told police he heard gunfire before his front windshield shattered. Police said the vehicle's side and rear windshield were also hit by gunfire. Numerous shell casings were found on the freeway, police said.

The man said after the shooting another vehicle hit the rear of his SUV. Police are not sure if that was related to the shooting or because he was trying to pull over. The man got off the freeway in Emeryville and stopped at a gas station to call police. He was taken by ambulance to a hospital for treatment and one of the passengers drove the SUV.

Police and Crime Stoppers of Oakland are offering up to $5,000 in reward money for information leading to the arrest of the gunman. Anyone with information may call police at 510-238-3426 or Crime Stoppers at 510-777-8572.