Three uniformed staff members at Rikers Island have been suspended following the death of a 40-year-old man in department custody, officials confirmed Wednesday.

Michael Nieves attempted suicide Aug. 25, according to a source, and was pronounced dead at Elmhurst Hospital just after 9:45 p.m. Tuesday. He was housed in a unit for detainnees with severe mental illness, the source said.

An officer stood outside the cell for 10 minutes and never rendered first aid while waiting for medical staff to arrive, the source added.

The Department of Correction has not publicly commented on the circumstances of Nieves' death, but DOC Commissioner Louis Molina confirmed three uniformed staff members at the prison were suspended following a “preliminary investigation” into the incident.

“The reports that New York City’s uniformed correction officers stood idly and watched Michael Nieves, our client, end his life are infuriating and tragic, but not surprising,"  the Legal Aid Society said in a statement. "It is heartbreaking that a person with severe mental illness was not in a hospital receiving care, but was held behind bars at the mercy of such demonstrably callous and incompetent jailers."

"We demand the immediate release of the videos of the staff during this incident so that New Yorkers can truly understand what takes place in the jails, and so that this depraved response to a mental health crisis is not swept under the rug as have been countless others," the organization added.

Nieves’ cause of death remains under investigation, with confirmation pending from the city’s medical examiner, the DOC said. As in all DOC custody deaths, the state attorney general's office and the city's Department of Investigation will also launch inquiries into Nieves’ death.

“This is a painful loss. Losing a loved one who is incarcerated is traumatic, and we send our deepest condolences to Mr. Nieves’ family and all those he held dear,” Molina said in a statement Wednesday.

“Any death in custody is a tragedy and we will be investigating the circumstances surrounding this incident,” he added.

Nieves was taken into custody on June 8 on a first-degree burglary charge, the DOC said.

Nieves is the 13th person to die after being held on Rikers Island this year. The Department of Correction considers Nieves’ death to be the 12th of a person in custody this year, but a 13th inmate passed away days after he was granted compassionate release following a suicide attempt.