Attorney General Eric Holder has instructed the department's civil rights division to monitor the developments in Ferguson, the Justice Department said.
The FBI said it is assisting police in its investigation and will review the findings.
Benjamin Crump, the lawyer who handled Trayvon Martin's case, will represent the family. Martin, 17, was killed in 2012 by a Florida neighborhood watch organizer, who was acquitted of murder charges. Ferguson, which has a population of about 21,000 people, is 67% African-American.