“This is really an ambush. You can’t see it coming,” St. Louis County Police Chief Jon Belmar told a press conference on Thursday. “You’re basically defenseless. That’s something very difficult to guard against when you have officers standing in a large group.”
Police outside the department provided similar security Thursday night, Al Jazeera reported.
About 100 people demonstrated again outside the police station late on Thursday, some banging drums while other chanted "If we don't get no justice, they won't get no peace!"
One woman wore a T-shirt that read: "Is my son next?"
Some demanded the resignation of Ferguson Mayor James Knowles.
The crowd blocked traffic at times and was told to clear the road by police using loudspeakers, but there were no arrests and by midnight local time the protesters were gone.
Activists who condemned the shooting had also earlier held a candle-light prayer vigil for peace, which was attended by about 40 people a short distance up the road.
"We deplore all forms of violence," said Reverend Osagyefo Sekou, who was in the crowd when shots rang out. "But we also deplore the findings of the Department of Justice report and the suffering and the misery that this community has endured."
The gatherings came after a day marked by St. Louis County investigators canvassing streets near the police station, peering into trash cans, down drains, and quizzing residents about what they saw or heard.
Several people were brought in for questioning, but all of them were later released and no arrests were made, police said. Two Missouri congressman offered a $3,000 reward for information leading to the culprit.