A Miami city police officer was found shot to death in his unmarked patrol car early Tuesday, authorities said.

Miami police Detective James Walker, 30, was found in an alley by North Miami Beach officers responding to reports of shots fired around 12:30 a.m., said Miami police spokesman Lt. Bill Schwartz. It wasn't known if Walker returned fire, he said.

Police were now searching for a suspect in a white Ford Taurus with bullet holes, but they didn't release details of the gunman. Blood trails led from Walker's car, where he apparently had been shot with a high-powered weapon, Schwartz said.

"His car had been filled with holes," Schwartz said.

One man being questioned had been shot in the arm Tuesday morning, but police did not call him a suspect. "He might just be an innocent bystander," Schwartz said.

Walker went off-duty around 11 p.m. Monday. It's not clear what he was doing in North Miami Beach.

Walker had been a member of the Miami police since 2000, and had been working with the department's domestic violence unit.

Walker's ex-wife lived in the area where he was found, said John Randle, the detective's brother-in-law.

"I don't know what he was doing in this alley," Randle said. "He was a good, humble guy. He was no trouble. I can't tell you no flaws."

Walker's body was taken to the Miami-Dade County Medical Examiner's office for an autopsy. The car was to be removed from the scene and examined for evidence separately, officials said.

North Miami Beach police referred calls to the Miami police department.

Walker's death is the fourth fatal shooting of a South Florida officer in the past six months. A Miami-Dade police officer was fatally shot in shootout with a suspect in September. One Broward sheriff's deputy was fatally shot in August while looking for stolen vehicles behind a drug store, and another was shot with his own weapon while transporting an inmate to court in November.