Three US prison guards are facing criminal charges over the death of an mentally-ill inmate who died of dehydration after water to his cell was cut off for a week, prosecutors say.

Key points: Prosecutors say Terrill Thomas had his cell water supply cut off as punishment

Prosecutors say Terrill Thomas had his cell water supply cut off as punishment He died of dehydration after a week

He died of dehydration after a week The prison guards responsible may face up to three-and-a-half to four years of prison

The charges announced by the Milwaukee District Attorney's Office came almost a year after jurors recommended jail staff should be held responsible.

An investigation revealed Terrill Thomas, 38 had water to his cell shut off as punishment for flooding it in April 2016.

Prosecutors say supervisor Kashka Meadors ordered jail deputy James Lee Ramsey-Guy to shut off the water.

Ms Meadors and Mr Ramsey-Guy are both charged with neglect of a resident of a penal facility, a felony that is punishable by up to three-and-a-half years in prison.

Jail commander Nancy Lee Evans is charged with felony misconduct and misdemeanour obstructing an office.

Detectives had asked her whether there was any documentation showing whether the water was turned off the whole time or intermittently.

The document Ms Evans signed off on answered that "there is no documentation indicating this."

However, prosecutors say Ms Evans was aware of the tape and that it showed the water to Mr Thomas' cell being turned off and never turned back on again.

Prosecutors say she failed to preserve surveillance video showing the water turned off and lied to police about what the footage showed.

Ms Evans could face more than four years in prison if she is convicted of both charges.

Dead man was in custody after 'breakdown' and shooting spree

When Mr Thomas died the jail was overseen by conservative firebrand Sheriff David Clarke, who resigned in August to join a political action committee in support of President Donald Trump.

Mr Clarke was not the target of the inquest because he was not directly involved in Mr Thomas' death.

Mr Thomas' family said he was having a mental breakdown when police arrested him on April 14, 2016, for shooting a man in front of his parents' house and later firing a gun inside a casino.

"Our expectation when Mr Thomas was brought into custody was that he would be kept safe. The allegations set forth in the criminal complaint document that that did not happen," District Attorney John Chisholm said.

"My heart bleeds for these people," said interim Sheriff Richard Schmidt, who was not in charge when Mr Thomas died.

"I'm a human being, I have family members. If this turns out to be exactly the way they've been written, it's horrific."

Jail also cut water to other inmates

The case was not the only time an inmate was punished by being deprived of water.

Prosecutors presented jurors with jail logs documenting two cases in which disobeying inmates had water to their cells turned off, both within a month of Mr Thomas dying.

One of the cases happened a week after his death.

Sheriff Schmidt said the three guards who had been charged had been suspended.

He said he would determine if disciplinary actions were needed by the end of the week, as his office conducted its own separate investigation.

Jurors had also recommended charges against four other guards — JorDan Johnson, Thomas Laine, Dominique Smith and John Weber — but Mr Chisholm concluded they should not be prosecuted.

He said, however, that later charges could be laid against the company contracted by the jail to provide health care to inmates.

AP