Chicago:Police Monday charged two African Americans with the "hateful murder" of a Hispanic off-duty officer, shot to death over the weekend in a city fighting to control gun violence.

Authorities charged Menelik Jackson, 24, and Jovan Battle, 32, with the shooting death Saturday of officer John Rivera, 23, in the city's tony downtown area. Police were still searching for a third suspect.

Rivera and a friend, who was wounded, were gunned down in their car - seemingly picked at random after Jackson and Battle failed to locate a group of Hispanic men with whom they had fought earlier in the evening, police said.

"In an act of cowardice, Mr. Jackson went to get a gun to settle this petty dispute, which resulted in him murdering the first Hispanic man that he came in contact with," Chicago police chief Eddie Johnson told a news conference.

Both suspects were known to police, with a history of criminal convictions. They face one count each of first degree murder and three counts of attempted murder.

Johnson, who called the crime a "hateful murder," said hate crimes charges were possible.

Rivera's supervising commander said colleagues were "struggling" with his death.

"He was such a good person - not only a good police officer, but a good person," Commander William Bradley said.

The killing came at a time when police have been making progress in combating a gun violence epidemic that has made the city one of the most violence-prone in the country.

More than 550 people were killed last year in the nation's third largest city - more than the combined total in New York and Los Angeles, both with greater populations.

So far this year, 60 people have been murdered, a 39 per cent reduction compared to last year.