Mr. Brown’s family and the authorities in Ferguson offered support for the federal investigation.

“We believe that transparency in law enforcement is the only way to build trust in the community, not just in the killing of Michael Brown, but for others who have suffered as well,” Benjamin L. Crump, a lawyer representing the Brown family, said in a statement.

Officials in Ferguson, in a statement, said they had met with Justice Department officials “and feel our collaborative efforts are another step forward in showing our willingness to be transparent and forthright as we continue the process of earning back the trust of our residents and our neighbors in the St. Louis region.”

Ron Hosko, the president of the Law Enforcement Legal Defense Fund, a nonprofit organization that defends police departments and officers in court cases and in public, criticized the investigation as politically motivated. Mr. Hosko, a former assistant director for the F.B.I., said the decision suggested that the Justice Department had already made up its mind that the police had violated Ferguson residents’ civil rights.

“Such a conclusion can only have a chilling effect on the thousands of law enforcement professionals who put their lives on the line every day to protect our communities,” Mr. Hosko said, adding that the large increase in civil rights investigations represented “a disturbing and growing trend.”

The broad federal review is unrelated to separate investigations by the county and the F.B.I. into the death of Mr. Brown, who was shot to death by Officer Darren Wilson in a confrontation on a Ferguson street on Aug. 9.