An investigation was pending Monday into the death of a 77-year-old Orange County Jail inmate.

The inmate, whose name was not immediately released, was taken to a hospital on Saturday to be treated for respiratory issues, officials said. The inmate was pronounced dead at the hospital Sunday.

The inmate, who was being housed in the Central Men’s Jail in Santa Ana, was booked on Dec. 17 by Tustin police on suspicion of criminal threats and brandishing a gun, officials said.

The inmate’s cause of death was pending and the Orange County District Attorney’s Office will investigate the in-custody death.