Those hoping the federal government will criminally prosecute Ferguson, Mo., police Officer Darren Wilson in the killing of an unarmed black man are likely to be disappointed, but chances are strong that the Justice Department will impose significant reforms on the city's police department through its ongoing civil investigation.

St. Louis County officials announced Monday that a local grand jury had found insufficient evidence to charge Wilson in the Aug. 9 shooting of Michael Brown, 18.

Some Ferguson activists hope that the Justice Department, which is conducting its own inquiry into the shooting, will file a federal criminal case against Wilson. But former civil rights prosecutors say the threshold for charging him with a federal crime is even higher than for local prosecutors because it requires proof that the officer intentionally used more force than reasonably necessary to deprive someone of his civil rights.

"It's a very tough thing under federal law to indict a police officer in a shooting," said William Yeomans, who supervised police investigations while serving as a top Justice Department official and currently teaches law at American University. "It doesn't happen very often."

Yeomans advised that Brown supporters not "put all their eggs in the criminal basket" by focusing too heavily on the outcome of the federal criminal investigation into Wilson. "The federal civil investigation into the Ferguson Police Department will continue and has the potential to result in some very significant reforms," Yeomans said.