CLAYTON, Mo. -- U.S. Attorney General Eric Holder on Wednesday told community leaders in Ferguson, Missouri, that he has assigned the federal government's "most experienced agents and prosecutors" to the investigation of a white police officer's fatal shooting of an 18-year-old African-American man.

Holder also reassured college students that "change is coming" after days of furious protests. He said federal prosecutors will be aggressive in learning the facts behind Michael Brown's shooting death Aug. 9.

Meanwhile, a grand jury was expected to begin hearing evidence to determine whether the officer should be charged.

Outside the St. Louis County Justice Center, where the grand jury was expected to convene, two dozen protesters gathered in a circle for a prayer, chanted, and held signs urging prosecutor Bob McCulloch to step aside. Nearly two dozen officers guarded the building's main entrance.

McCulloch's deep family connections to police have been cited by some black leaders who question his ability to be impartial in the case of Darren Wilson -- the white officer who fatally shot Brown. McCulloch's father, mother, brother, uncle and cousin all worked for the St. Louis Police Department, and his father was killed while responding to a call involving a black suspect.

Missouri Gov. Jay Nixon has said he would not seek McCulloch's removal from the case.

The prosecutor, who is white, has insisted his background will have no bearing on the handling of the Brown case, which has touched off days of nighttime protests during which authorities used tear gas and rubber bullets to clear the streets.

The protests were more subdued Tuesday night. Police said they still made 47 arrests, but mainly of people who defied orders to disperse.

Holder was expected to meet with FBI and other officials carrying out an independent federal investigation into Brown's death, as well as with community leaders. Holder arrived in St. Louis with several Justice Department officials including members of its Civil Rights division.

In a letter published late Tuesday on the St. Louis Post-Dispatch website, Holder promised a thorough investigation while calling for an end to the violence in Ferguson.

Arrest patterns "must not lead to disparate treatment under the law, even if such treatment is unintended. And police forces should reflect the diversity of the communities they serve," Holder wrote.

The Justice Department has mounted an unusually swift and aggressive response to Brown's death, from conducting an independent autopsy to sending dozens of FBI agents to Ferguson in search of witnesses to the shooting.

As for the county grand jury, prosecutor's spokesman Ed Magee said Wednesday that there is no timeline for how long the process could take, but it could be weeks.

Meanwhile, Brown's funeral arrangements were set. The Austin A. Layne Mortuary said the funeral will be Monday morniing. Brown's uncle, the Rev. Charles Ewing, will deliver the eulogy, and the Rev. Al Sharpton will also speak.

Associated Press writers Jim Suhr in Clayton, Alan Scher Zagier in Florissant, Jim Salter in St. Louis and David A. Lieb in Jefferson City contributed to this report.