Giancarlo Scotti, a 10-year veteran of the department, was arrested at 6 p.m. Wednesday, within hours of two female inmates coming forward with the allegations at the Century Regional Detention Facility, a women's jail in Lynwood.
An investigation began at about 9 a.m., when a female inmate reported the crime to an instructor who works at the facility. The woman said she was assaulted by Scotti in one of the dormitories.
A second female inmate also complained about a sexual assault, both which were alleged to have occurred during the early morning hours while the deputy was on duty, sheriff's officials said.
Within five hours of the first complaint, the scene was locked down to preserve potential evidence. The Internal Criminal Investigations Bureau and a scientific services team conducted an investigation while the deputy was placed in the watch commander's office.
As a result of "compelling evidence," Scotti was arrested.
Scotti, 31, faces two counts of rape and two counts of oral copulation -- both under color of authority.
McDonnell could not say whether there were prior issues with Scotti but called the allegations "disgusting" and said it was important to address the issue as soon as possible.
"For something like this to happen, certainly, we take exceptionally seriously and that's really why we're here today," he said.
Scotti posted $100,000 bail and has been placed on administrative leave. He is set to appear in court on Oct. 5.
McDonnell said there is no indication or evidence any other personnel were involved in the crimes.