FERGUSON (REUTERS) - US Attorney General Eric Holder met with community members in Ferguson, Missouri, on Wednesday and vowed a thorough civil rights probe into the fatal police shooting of an unarmed black teenager that has set off 11 nights of racially charged unrest.

Holder, the first African-American to head the Justice Department, met with students and then community leaders at a community college during a visit to Ferguson for a briefing on a Justice Department investigation into the Aug 9 killing of 18-year-old Michael Brown.

He later met privately with Brown's parents at the St. Louis US Attorney's Office, but no details of that session were immediately available.

Also on Wednesday, a grand jury investigating the fatal shooting began hearing evidence in the case.

Before a briefing at local FBI headquarters, Holder said the thrust of his department's inquiry differed from the investigation conducted by local authorities. "We are looking for violations of federal, criminal civil rights statutes," he said.

The Justice Department probe specifically seeks to determine whether federal prosecutors can bring criminal charges against Darren Wilson, the police officer who shot Brown, for violating Brown's civil rights by use of excessive force.

His visit came hours after dozens of protesters were arrested in the latest street disturbances since the shooting. Many of the protests have been peaceful, but others, especially smaller ones late at night, have been punctuated by looting, vandalism and clashes between demonstrators and police.

The turmoil has cast the St. Louis suburb of 21,000 people into the international spotlight as a symbol of often troubled US race relations. Ferguson is predominantly black, but its police force, political leadership and public education administration are dominated by whites. Activists and demonstrators have complained that Brown's death was the culmination of years of unfair police targeting of blacks.

An hour before sundown on Wednesday, a few dozen protesters began marching peacefully along a main thoroughfare that has been the scene of nightly demonstrations and sporadic violence. The group chanted: "Hands up, don't shoot," which has become demonstrators' rallying cry, along a stretch of avenue fronted by businesses with boarded-up windows and the ruins of a gasoline station burned out in a previous night of unrest.

But heavy clouds brewing over Ferguson, accompanied by gusting winds and flashes of lightning in the distance, portended the possibility of protests being cut short. Among students meeting with Holder at St. Louis Community College was Molyric Welch, 27, who said her brother died three years ago after Ferguson police used a stun gun on him.

"A lot has happened here," she said. "He (Holder) promised things were going to change."

But Melvin Brown, 61, a pastor visiting Ferguson from Rockford, Illinois, to support the protests, was skeptical. "I don't have any faith in it," he said. "The whole system is corrupt."

Grand Jury

St. Louis County Prosecuting Attorney Bob McCulloch said his office could continue presenting evidence to the grand jury - which meets once a week - through mid-October as he confronts conflicting pressures for speed and thoroughness.

"On one side, people are saying: 'You're rushing to justice,' and on the other side, they're saying: 'You're dragging this thing out,'" he told a news conference.

"We're going to present this as expeditiously as possible, but we are not going to present it in a half-hearted manner."

Outside McCulloch's office, scores of protesters led by clergy members called for his removal from the case. They also demanded the appointment of a special prosecutor, an expedited grand jury proceeding and the immediate arrest of the 28-year-old officer involved in the shooting.

Wilson has been placed on leave and gone into seclusion.

McCulloch's father was a police officer killed in the line of duty by a black man, and his critics say he has a record of discriminatory handling of cases involving police and suspected mistreatment of blacks.

"There is no trust in Bob McCulloch," said Clinton Stancil, senior pastor of Wayman AME Church in St. Louis. "We are seeking justice. We don't think he can be fair."

McCulloch has repeatedly promised a fair and impartial investigation.

Accounts of Brown's slaying differ. According to police, Wilson reported that Brown reached into the policeman's cruiser when Wilson approached him on the street, then grabbed for the officer's gun. A companion of Brown said the teenager was initially shot after the officer tried to grab him through the car window and again after Brown staggered back with his hands in the air.

Eyes of the world

Holder said during his visit that he had assigned the most experienced agents and prosecutors to the investigation. Hundreds of people have already been interviewed and federal medical examiners have performed an independent autopsy, the third conducted in the killing.

The case has reignited a national debate over racial disparities in the US criminal justice system and has drawn sharp words from top UN officials and human rights groups about police tactics and the need to respect the rights of protesters.

Police say thugs and outside agitators have caused most of the trouble at the nightly demonstrations. On Tuesday, demonstrators were noticeably fewer in number and more subdued than on previous nights, but the relative calm dissolved just before midnight, as police in riot gear ordered lingering demonstrators to disperse, then charged into the crowd to make arrests.

Police said later they took 47 people into custody and seized several loaded firearms, but no gunshots were fired, despite extremely tense moments. One officer from the neighbouring community of St. Ann was suspended from duty for pointing a semi-automatic assault rifle at a peaceful protester during a heated verbal exchange, before a police sergeant stepped in, forced the officer to lower his weapon and escorted him away from the scene, the St. Louis County Police Department said.