The families of at least four unarmed black men killed this year by police are pinning their hopes on possible federal prosecutions under a little-known civil rights statute that has only occasionally been used to indict officers in such cases.

The Department of Justice (DOJ), the FBI and local US attorneys’ offices have for weeks been examining the deaths of Michael Brown, Eric Garner and John Crawford III – all of whom were killed by police officers who grand juries later decided not to indict on state criminal charges.

Late on Monday, the DOJ announced it would review a fourth death: that of Dontre Hamilton, who was shot 14 times by a police officer following a confrontation in a park in Milwaukee, Wisconsin, in April.

The review was launched after the Milwaukee district attorney announced that Christopher Manney – the police officer who shot Hamilton, and who has since been fired – will not face charges.

In all four cases, federal officials are considering whether there is sufficient evidence to bring charges under a section of the US code that prohibits public officials from depriving an individual of constitutional rights under “color of law”.

The DOJ did not respond to questions about whether there were any additional police shooting cases from 2014 – aside from those relating to the deaths of Brown, Garner, Crawford III and Hamilton – that are currently subject to departmental review or investigation.

However, officials are not believed to be looking into the case of Darrien Hunt, a black man who was killed by police in Saratoga Springs, Utah. They are awaiting the outcome of the local investigation into the shooting of Tamir Rice, a black 12-year-old killed by police as he played with a plastic gun in a park in Cleveland.

Police officers, prison guards, judges and care providers at public health facilities can all be prosecuted under the title 18, section 242 of the US code, more commonly known as the “color of law” statute.

John Crawford III with his mother, Tressa Sherrod. Photograph: AP

The government does not have to prove any racial bias for a successful prosecution and cases involving the death of a victim can theoretically be punished with life imprisonment or even a death sentence.

However experts caution that while the DOJ and FBI bring dozens of “color of law” cases to court each year – including some regarding deaths in custody – it is uncommon for prosecutions to be brought in police shooting cases.

One notable exception was the federal prosecution of five New Orleans police officers over the shooting of unarmed people on Danziger bridge following Hurricane Katrina in 2005, two of whom died. The officers were convicted but later acquitted by a federal judge who ordered a retrial.

Perhaps the best-known federal “color of law” prosecution was brought against the Los Angeles police officers acquitted of state criminal charges over the beating Rodney King in 1991.

William Yeomans, a law professor in Washington, DC and a former acting assistant attorney general for civil rights in the Justice Department, told the Guardian “color of law” prosecutions involving police shootings are harder to prove than other types of deprivation and are therefore “very rare”.

“There is always the possibility of a federal prosecution but I think everyone recognises it is not the most likely outcome because the standard is very high,” he said. “The government has to show that the officer acted with the specific intent to use more force than was reasonably necessary under the circumstances.”

Yeomans said state prosecutors have “options that the federal government does not have”. The spectrum of possible indictments in police shooting cases run from an intentional premeditated killing all the way down to the far less serious crime of involuntary manslaughter, where criminal intent does not need to be proven.

“If those options have already been examined by the state and the state has decided not to proceed, it is very difficult for the federal government to make a case where it has to show that the officer acted with the intent to use more force than was necessary,” he explained.

The DOJ said this week it would only “review” the evidence in the case of Hamilton, who was shot following an altercation with Manney in Red Arrow Park in April. That could however be expanded into a full investigation at a later date.

According to the Journal Sentinel, which reviewed newly released documents in the case, Hamilton had been lying in the park and had already been checked on by two other police officers, who deemed he was doing nothing wrong, before he was approached by Manney.

The police officer patted Hamilton down and told investigators he felt a cylindrical object in Hamilton’s waist band and another hard object in his pocket. The two men began to scuffle and Hamilton wrested away control of Manney’s baton, striking him on the neck.

Manney told investigators he felt Hamilton was displaying “super-human strength”. It was shortly afterwards, he said, fearing for his life, that he shot Hamilton 14 times. No weapons were found on Hamilton’s body.

A memorial setup in the place where Michael Brown was killed in Canfield apartments in Ferguson, Missouri. Photograph: Michael B Thomas/AFP/Getty Images

The DOJ, FBI and US attorney’s office review is the same tier of inquiry used in the case of John Crawford III, who was fatally shot by police in August after he was seen carrying a BB rifle in a Walmart store in Dayton, Ohio.

Full-scale federal inquiries, in contrast, have been launched into the police shooting of Michael Brown in Ferguson, Missouri, in August, and the fatal encounter Eric Garner had with the New York Police Department in Staten Island in July.

The federal probe into the Brown case was announced by US attorney general Eric Holder just days after his death, which sparked protests and riots in the suburb of St Louis. For several months, the federal civil rights investigation ran alongside the state criminal investigation.

The state investigation concluded in November, after a grand jury decided not to bring charges against the officer involved, Darren Wilson, prompting more rioting. But the federal investigation is still ongoing.

The Brown case was unusual because state and federal investigations do not typically run in parallel. Federal investigations have traditionally been used as a backstop mechanism that is only implemented when state inquiries have run their course.

The federal inquiry in the Garner case, for example, was only initiated after the grand jury decided not to charge the officer, Daniel Pantaleo, who had been filmed placing Garner in a chokehold.

Both the Crawford and Hamilton reviews, and the Brown and Garner investigations, could continue for several more months or even years.

The DOJ investigation into George Zimmerman, the night watchman who shot dead an unarmed black teenager, Trayvon Martin, in Florida, is being run out of the same civil rights division as the four “color of law” cases.

The Zimmerman case, which is understood to relate to a different federal statute that prohibits the use of force because of racial discrimination, has been ongoing since March 2012.