LOS ANGELES - Two men opened fire on a police car patrolling a tough part of Los Angeles, but the two officers inside were not injured and one was able to shoot back, authorities said Monday. One of the suspects was later arrested and the other is on the loose.

The shooting occurred amid heightened tensions over attacks on police elsewhere in the US after grand juries declined to indict white officers in the killings of unarmed black men.

Police have not yet determined a motive for the Sunday night shooting in South Los Angeles - an area plagued by gang violence - but said there were no indications it was linked to other attacks on police in the country.

"It was a complete unprovoked attack," LAPD Deputy Chief Bob Green said initially. "They were just driving on the street, and somebody struck out and tried to kill two policemen."

However, Green later said police also were looking into whether the officers might have driven into an ongoing incident and were inadvertently fired at.

"Last night the initial assessment, based on the reaction of officers and the adrenaline factor, was they were getting ambushed, but, you know, things change and it takes a lot to investigate this," Green said.

Officers arrested one man shortly after the attack. Christopher Taylor, 18, was booked on suspicion of shooting at an unoccupied vehicle.

An hours-long search followed for the other man, but he remained at large. Police found a handgun and rifle at the scene, and were looking for video evidence and conducting interviews to determine details such as why and how the shooting occurred.