The US justice department has announced a civil rights investigation into the police department involved in the shooting of an unarmed black teenager by a white police officer.

The death of 18-year-old Michael Brown, from Ferguson in the state of Missouri, has already given rise to a separate federal investigation into whether Darren Wilson, who shot him, violated the teenager's civil rights.

Eric Holder, the attorney general, said on Thursday that the justice department had determined there was cause to investigate whether Ferguson officials had violated of the US constitution.

Holder said the investigation was "a long-term strategy" meant to build trust, safety and accountability.

Brown's shooting on August 9 and subsequent protests reopened a fierce national debate on race and the tactics used by police in dealing with minorities.

City leaders in Ferguson are pledged their full cooperation with the investigation into their police department.

Police chief Jon Belmar said he initiated the "collaborative reform'' and "welcomes the scrutiny", the AP news agency reported.

A grand jury is also hearing evidence to determine whether Wilson used excessive force by shooting Brown six times, killing him.

Brown's family said it was encouraged by the federal investigation, and urged the use of body cameras for police around the country.

"We can't have another young man's life taken amid murky circumstances. We want the truth to shine brightly," the family's lawyer said.

Over the past five years, the justice department has conducted similar investigations in 20 police departments and is helping enforce 14 agreements to reform practices across the US, Holder said.