FERGUSON, Mo. — An 18-year-old black man shot multiple times by a suburban St. Louis police officer was unarmed when he died, police said Sunday, as hundreds of local residents protested and a civil rights leader expressed outrage at the killing.

Michael Brown had graduated from high school and was about to enter a local college, said his mother, Lesley McSpadden.

St. Louis County Police Chief Jon Belmar said the shooting occurred after an officer encountered two people – one of whom was Brown – on the street near an apartment complex Saturday afternoon in Ferguson, a predominantly black suburb a few miles north of downtown St. Louis. Belmar said one of the men pushed the officer back into his squad car and a struggle began. Belmar said at least one shot was fired from the officer’s gun inside the police car. Ferguson Police Chief Tom Jackson said authorities were still sorting out what happened inside the police car. It was not clear if Brown was the man who struggled with the officer.

The struggle spilled out into the street, where Brown was shot multiple times. Belmar said the exact number of shots wasn’t known, but “it was more than just a couple.” Police are still investigating why the officer shot Brown.

Jackson said the second person has not been arrested or charged and was expected to be interviewed later Sunday. Authorities aren’t sure if that person was unarmed, Jackson said.

McSpadden said she doesn’t understand why police didn’t subdue her son with a club or Taser instead of shooting him, and she said the officer involved should be fired and prosecuted.

The killing drew criticism from some civil rights leaders, and they referred to the 2012 racially charged shooting of 17-year-old Trayvon Martin by a Florida neighborhood watch organizer who was acquitted of murder charges, as well as the New York City man who died from a police chokehold.

“We’re outraged because yet again a young African-American man has been killed by law enforcement,” said John Gaskin, who serves on both the St. Louis County and national boards of directors for the NAACP.

The Rev. Al Sharpton called the shooting death “very disturbing” and the New York-based civil rights leader said he planned to go to Ferguson to meet with the family Monday night or Tuesday.

A few hundred protesters gathered outside Ferguson Police headquarters about the time the news conference was to begin. At one point, many of them marched into an adjacent police building, some chanting “Don’t shoot me” while holding their hands in the air. Officers stood at the top of a staircase, but didn’t use force; the crowd eventually left.