FERGUSON, Mo. -- Crews erected barricades Saturday around the building where a grand jury has been considering whether to indict a white police officer in the shooting death of an unarmed black 18-year-old, even as a grand jury decision seemed unlikely this weekend.

Tension has been mounting in Ferguson and elsewhere in the St. Louis area in recent days, with many speculating that the grand jury's decision would be announced on Sunday. That seemed increasingly unlikely by Saturday afternoon, although there was a noticeable uptick in the preparations being made.

The grand jury is weighing whether to indict Ferguson Police Officer Darren Wilson, who killed Michael Brown, during a violent confrontation on Aug. 9. The killing of Brown led to protests, some of which turned violent, and drew international attention.

Brown's killing reignited a debate over how police treat young black men. It drew attention to racial tensions simmering in Ferguson and other U.S. communities four decades after the civil rights movement of the 1960s. Two-thirds of Ferguson's residents are black but the police force is almost entirely white.

Police arrested three protesters on Friday night -- the third straight night of unrest in Ferguson.

Wilson, 28, reportedly told the grand jury that he feared for his life as Brown, who was 6-foot-4 (1.93-meters) and nearly 300 pounds (136 kilograms), came at him. Some witnesses said Brown was trying to surrender and had his hands up.

Downtown STL Inc., a St. Louis civic group that promotes downtown businesses, told members in an email Saturday that the grand jury will reconvene Monday to continue deliberating whether charges are warranted against Wilson.

The email did not explain how the group knew the information, and a spokeswoman declined comment. Ed Magee, a spokesman for St. Louis County Prosecutor Bob McCulloch, didn't respond to several messages Saturday.

The Brown family's attorney, Ben Crump, said Saturday that he hadn't heard a decision had been reached and that prosecutors had promised to tell him when that happened.

On Saturday, authorities set up barricades around the Buzz Westfall Justice Center in Clayton, which is where the St. Louis County grand jury has been meeting.

Businesses in both Ferguson and Clayton have put boards on their windows.

Residents were on edge, too.

Jamie Freeman of Ferguson, 38, a registered nurse and mother of four, said she was especially concerned since her 20-year-old son lives in the neighbourhood where Brown was shot.

"I just hope it stays peaceful," Freeman said of protests that will follow the grand jury decision. "We all have human emotions, but there's a way to do things, and violence, you can't get peace from violence."

Crump, the Brown family attorney, said the grand jury process is weighted against those shot by police officers.

"Ninety-nine per cent of the time the police officer is not held accountable for killing a young black boy," Crump said. "The police officer gets all the consideration."

The FBI has sent nearly 100 additional agents to Ferguson to help law enforcement agencies, according to a U.S. official who spoke on condition of anonymity because the official was not authorized to discuss the FBI plans.

But things were calm during the day on Saturday. Brown's father, Michael Brown Sr., joined a church group in passing out free turkeys to needy residents in the area where his son was shot. A day earlier, a video of Brown Sr. was released urging peace, regardless of how the announcement goes.