FERGUSON, Mo. — A Missouri state prosecutor on Tuesday prepared to present evidence to a grand jury in the police shooting death of an unarmed black teenager, hours before Attorney General Eric Holder Jr. was set to arrive to personally oversee the federal investigation.

A spokesman for St. Louis County Prosecutor Robert McCulloch said a grand jury planned to begin hearing evidence Wednesday in the shooting of 18-year-old Michael Brown by Darren Wilson, a white Ferguson police officer. It remained unclear whether Wilson would face charges in the Aug. 9 incident, which has triggered days of violent protest and dozens of arrests.

“We know this is of interest to a lot of people around the country,” the spokesman, Edward Magee, said in an interview. “We’re going to do this fairly and also attempt to do it in a timely manner.”

In the nearby suburb of Clayton, a small group of demonstrators who had gathered across the street from McCulloch’s office grew in size and fervor Tuesday. Chanting “No justice, no peace, no racist police,” the crowd rushed to the glass-front atrium of the county office building and was met by a wall of heavily armed police officers. At least two people were arrested.

In all, more than 78 people have been arrested in Ferguson since the protests began, according to St. Louis County police. About 40 of them were arrested on Monday night alone, as small groups of demonstrators faced off with officers firing tear gas. Two people were shot in Ferguson during the chaotic night, apparently by others in the crowd – police said no officers fired their weapons.

The St. Louis county executive and other local black leaders have challenged McCulloch’s fitness to handle the case, in part because his father, a police officer, was killed in the line of duty when McCulloch was 12 years old. The man who shot his father was black.

The county executive, Charlie Dooley, has said he also feels that McCulloch acted inappropriately when he publicly criticized Missouri Gov. Jay Nixon’s decision to bring in the state highway patrol to lead efforts to quell the violent street protests that erupted after Brown’s death.

McCulloch has declined to step aside and has said his father’s death does not affect his judgment.

His investigation of Brown’s death is being monitored by the Justice Department and the FBI, which are also investigating the shooting in an expanding federal probe that has yielded more than 200 interviews. Holder will arrive in Ferguson on Wednesday to meet with federal prosecutors and agents.

The new developments came on a day in which the first public indications emerged of Wilson’s version of events on that fateful day. The officer, who is on paid administrative leave and whose whereabouts are unknown, has told investigators that he struggled with Brown in his police cruiser and opened fire out of fear for his life after Brown charged at him, according to people familiar with Wilson’s account.