Randall Walker, the assistant Sonoma County sheriff and longtime commander of the jail system who was put on paid administrative leave three months ago amid an internal investigation into an undisclosed personnel matter, is retiring, sheriff’s officials said Thursday.

Walker, 52, notified the Sheriff’s Office of his decision in mid-June, and his retirement is set to become effective Sept. 24, according to the agency.

Capt. Al Vernon, a 27-year veteran of the office, will become the new assistant sheriff responsible for overseeing the county jail system. He begins in the new post on Tuesday.

The shakeup comes after Walker was placed on paid leave April 4, a move Sheriff Rob Giordano later described as motivated by a “recent personnel issue” that prompted an investigation from the Sheriff’s Office internal affairs division. That investigation is not yet complete, according to Sheriff’s spokesman Sgt. Spencer Crum.

Crum said he couldn’t speak to why Walker chose to retire. Crum referred those questions directly to Walker, who couldn’t be reached for comment Thursday.

“All we can say is that he voluntarily decided to retire,” Crum said. “We wish him the best in retirement.”

Walker will stay on paid leave until his retirement is effective, according to Crum.

The announcement of Walker’s retirement comes about one month after Sonoma County agreed to pay $1.7 million to settle a federal lawsuit from former jail inmates who alleged physical assault and verbal abuse from correctional deputies that started with a May 28, 2015, incident at the main detention facility in Santa Rosa.

The attorney for the seven former inmates who filed the suit has said as many as 20 inmates were beaten as deputies followed a sheriff’s policy known as “yard counseling” or “behavior counseling” that was established two decades ago by Walker.

Sheriff-elect Mark Essick has said he will do away with procedures depicted on videos from the case when he takes office next year.

Walker was hired in 1991 as a correctional deputy at the Sonoma County Jaill, after having served in the Army for nine years. Former Sheriff Steve Freitas appointed him asto assistant sheriff in 2012.

As Walker’s replacement, Vernon will oversee the county’s two jails: the main adult facility in Santa Rosa and the minimum-security North County Detention Facility near the county airport. The jail system employs 285 people and housed 1,100 inmates as of Thursday.

Vernon, 58, also joined the Sheriff’s Office in 1991, when he started working as a civilian sSheriff’s aide. He became a correctional deputy in 1992 and served in that capacity until 1996, when he joined the law enforcement division.

Vernon rose through the ranks after that, earning a promotion to captain in 2017. He has been in command of the patrol division for the past year.

As assistant sheriff, Vernon’s base salary will be $156,262, Crum said. Walker earned $165,247 last year, including overtime pay and other earnings, according to county payroll records.

Giordano, in a statement, praised Vernon’s experience in both detention and patrol.

The sheriff called Vernon a a “a true leader and role model” for the entire office and someone who is “well respected by staff from both law enforcement and detention alike.,”

the statement said.