FBI and Justice Department investigators from St. Louis and Washington have already spoken with key witnesses who saw a police officer shoot an unarmed teenager during a confrontation last weekend in Ferguson, Mo., Atty. Gen. Eric H. Holder Jr. said Thursday.

He said the Justice Department is pushing ahead with a possible federal criminal prosecution in the slaying of 18-year-old Michael Brown, saying it would be “thorough and fair,” but cautioning that the review would “take time to conduct.”

Though Holder did not discuss the substance of the witness interviews, his statement was a sharp departure from the silence of local officials who have refused to discuss any aspect of their investigation, declining to name the officer who fired the fatal shots.

Holder’s remarks also signaled that federal law enforcement officials may not wait for local authorities to act first, which traditionally happens when there are dual federal and state investigations.

Amid growing public pressure, Washington is moving forward even as local authorities continue to gather evidence to present to a local grand jury and await Brown’s autopsy and toxicology results, which could be weeks to complete. Often local investigations into police shootings take months to be resolved, and are ruled justified with little public accounting.

Holder also took aim at the images of heavily armed police officers using riot gear and military-style weapons against protesters.

“I am deeply concerned that the deployment of military equipment and vehicles sends a conflicting message,” Holder said in a statement released after he briefed President Obama.

“At my direction, [Justice] Department officials have conveyed these concerns to local authorities.”

Holder said the Justice Department has offered technical assistance to local officials “to help conduct crowd control and maintain public safety without relying on unnecessarily extreme displays of force.” He said local authorities in Missouri accepted the offer.

Holder also condemned looters and rioters. “While the vast majority of protests have been peaceful, acts of violence by members of the public cannot be condoned,” he said.

He criticized “willful efforts to antagonize law enforcement officers,” saying “such conduct is unacceptable.”

Further, Holder said members of the department’s Community Relations Service have been dispatched to Ferguson to meet with city, religious and neighborhood officials. Their aim, Holder said, is “to plot out steps to reduce tensions in the community.”

Holder’s actions came after calls from lawmakers in Washington for a stronger federal response.

“We need to demilitarize the situation,” said Sen. Claire McCaskill (D-Mo.) “This kind of response by the police has become the problem instead of the solution.... My constituents are allowed to have peaceful protests.”

Others suggested stronger measures to quell the violence. Rep. John Lewis (D-Ga.), a veteran of the civil rights protests of the 1960s, noted that President Kennedy federalized the National Guard and declared martial law in Montgomery, Ala., to protect demonstrators.

The uproar in Ferguson, Lewis said, “is a good time” to consider the words of Dr. Martin Luther King Jr.: “Peace is not the absence of conflict, but the presence of justice.”

richard.Serrano@latimes.com

@RickSerranoLAT