The second person has not been arrested or charged, and it was not clear if he was armed, Jackson said. Blood samples were taken from Brown and the officer who shot him for toxicology tests, which can take weeks to complete.
Earlier Sunday, a few hundred protesters gathered outside Ferguson police headquarters. Some marched into an adjacent police building chanting "Don't shoot me" while holding their hands in the air. Officers stood at the top of a staircase until the crowd eventually left. A similar protest that attracted about 250 people was held Monday morning.
Brown's mother, Lesley McSpadden, said she did not understand why police did not subdue her son with a club or stun gun. She said the officer involved should be fired and prosecuted.
"I would like to see him go to jail with the death penalty," she said, fighting back tears.
The killing drew criticism from some civil rights leaders, who 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.
Brown's family planned to speak later Monday at a news conference with their attorney, Benjamin Crump, who also represented Martin's family.
John Gaskin, who serves on both the St. Louis County and national boards of directors for the NAACP, said the group was "outraged because yet again a young African-American man has been killed by law enforcement."
Ferguson's population of about 21,000 people is almost 70 percent black.
Associated Press Writer Jim Suhr in St. Louis contributed to this report.
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