The Missouri National Guard rolls into Ferguson, Mo., but stay back as protesters again tangle with police, who respond with tear gas. (Lee Powell/The Washington Post)

A Missouri state prosecutor on Tuesday prepared to present evidence to a grand jury in the police shooting death of an unarmed black teenager, as Attorney General Eric H. 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 chaotic confrontations between protesters and heavily armed police.

“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.”

As darkness fell Tuesday in Ferguson, police officers and protesters braced for a repeat of the late-night confrontations.

In the early evening, as usual, there was calm. The night began with a procession of people chanting “Hands up, don’t shoot.” A group of elementary-school girls handed out flowers. Police were dispersed throughout the crowd, instead of facing down protesters in a long line.

At that point, only some officers seemed to be enforcing a rule that protesters could walk but could not stand still or congregate.

“I saw the police arrest a lady because she had stopped walking,” said Bryan Maynard, 35, who had come to see the protests firsthand Tuesday. It was his first night on West Florissant Avenue, the suburban thoroughfare that has become the scene of repeated nighttime clashes. “It’s more than heavy-handed.”

But the worst time in Ferguson has never been the early evening.

“At some point every night, it’s like a switch is flipped. Sometimes a rock gets thrown. An unruly march. A Molotov cocktail,” said Jon Belmar, the St. Louis County police chief, early in the night.

Later, about 9:30 p.m. local time, hundreds of people looped up and down the street, alternating chants of “We ready!” and “No justice, No peace, No racist police” and “We are, Mike Brown.”

Near the center of the street stood Alerion Smith, a 6-year-old who waved in his hand a “know your rights” booklet.

His mother said that the other day he came up to her and said, unprompted: “Having your hands up means ‘Don’t shoot.’ It means surrender.” That was when she decided he was ready to see the protests for himself.

Angry aftermath of the Missouri shooting

“Obvious this is something he had heard about and was affecting him,” said Alexis Simpson, 29. “So I decided to bring him out to see what is going on in his own neighborhood.”

About 10 p.m. local time, TV cameras showed officers making arrests. One night earlier, on Monday evening, at least 59 people had been arrested in Ferguson, according to figures from the St. Louis County jail. All but three were charged with “refusal to disperse,” a misdemeanor. Of the others, one was charged with “interfering with an officer. And two were charged with “unlawful use of a weapon,” a broad charge that could mean firing a weapon or simply exhibiting it in “an angry or threatening manner.”

Most of those arrested Monday — 41 of the 59 — were from Missouri. The others came from as far away as Chicago and San Diego. Also on the rolls were at least five journalists, including three German reporters and two Americans, a reporter and a photographer.

On Tuesday — even before the grand jury had heard evidence about the shooting — it was obvious that there was enormous political pressure on prosecutors in this case.

In the afternoon, a small group of protesters gathered outside the county prosecutor’s office in the suburb of Clayton, Mo., then rushed to the glass-front atrium of the county office building. They were met by a wall of heavily armed police officers. At least two people were arrested.

In addition, St. Louis County’s chief executive and other local black leaders have said they believe the county prosecutor is not fit to handle the case. That’s because McCulloch’s father, a police officer, was killed in the line of duty when the prosecutor was 12 years old. McCulloch is white. The man who shot his father was black.

McCulloch has declined to step aside and has said his father’s death will not affect his judgment. On Tuesday, Missouri Gov. Jay Nixon (D) said he has not asked McCulloch to recuse himself from the case.

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.

Wilson, the officer who shot Brown, is on paid administrative leave and has temporarily moved out of his home. He 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 interview with investigators.

Brown’s family has said their son was shot with his hands in the air in an execution-style slaying.

The Justice Department, which ordered its own autopsy of Brown’s body, has reached conclusions similar to those reached in two other autopsies, people familiar with the findings said Tuesday. Those autopsies, by the county medical examiner and by a medical examiner brought in by the teen’s family, concluded that Brown was shot six times.

At all levels, the citizens of Ferguson remain frustrated with law enforcement, Missouri state Rep. Sharon Pace (D) said Tuesday. Pace, who represents the section of Ferguson where Brown was killed, said county and federal officials need to publicly define their roles in the investigations and outline the next steps they will take.

“We aren’t getting any information,’’ Pace said in an interview. “What can we expect to happen and in what order will it happen? They need to tell us where we go from here. We don’t want to rush the process but we need information.”

Earlier in the day, police said they had come under “heavy gunfire” Monday in another night of violence in this battle-scarred community of 21,000 people.

At a news conference, Capt. Ron Johnson of the Missouri State Highway Patrol said officers refrained from firing back at the protesters. Numerous fires were also set, he said.

Johnson said Monday night’s two shooting victims were both men, but he had no information on their condition or identities. He stressed that “not a single bullet was fired by officers.”

Johnson also had harsh words for the news media, saying journalists have been failing to clear areas that police need to access. He also asked reporters not to “glamorize the acts of criminals.”

Standing in front of a table that displayed two handguns and a Molotov cocktail, Johnson said the weapons had been seized from “criminals” who were hiding behind those peacefully protesting.

“All of these criminals at night that are masking themselves and hiding themselves behind peace, let them come at night so we can identify them, so we can take them away from our community and put them away and make our streets clear,” Johnson said.

DeNeen L. Brown and Emily Wax-Thibodeaux in Ferguson and Kimberly Kindy, Julie Tate, David A. Fahrenthold and Adam Goldman in Washington contributed to this report.