FERGUSON, Mo. — The F.B.I. on Monday opened a civil rights inquiry into the fatal shooting by a police officer of an unarmed black teenager here as protests bubbled into a third night in this St. Louis suburb.

Officers donned riot gear and warily faced off against hundreds of people in response to looting and violence on Sunday night that resulted in 32 arrests.

On Monday night, police officers using tear gas and rubber bullets tried to disperse the crowd of mostly African-Americans, who had been gathering through the day under the hot sun. The protesters questioned the role that race — and simmering tensions between residents and the Police Department — may have played in the killing of Michael Brown, 18, who was to start college this week.

The standoff lasted for more than an hour, with about a dozen men approaching officers with their hands up saying, “Don’t shoot me.” At least 100 police officers were on the scene, shining bright lights into the crowd and telling people to return to their homes.