A Rikers Island inmate died Saturday afternoon, the Department of Correction said.

William Johnstone, 47, was found unresponsive in his cell unresponsive in the George R. Vierno Center at approximately 1:50 p.m., DOC officials said in a statement.

According to the DOC, he received immediate medical care and was transported to Mount Sinai Queens, where he was pronounced dead at 3:50 p.m.

Johnstone is the sixth person to die who spent time in city custody this year, and the third one this month.

Last year, 19 incarcerated people died in Rikers Island jails, a historic high in New York City.

Johnstone's cause of death is under investigation by the city's Office of the Chief Medical Examiner, officials said.

As is in the case of all deaths of people in city custody, the state Attorney General and the city's Department of Investigation will launch inquiries into Johnstone's death.