A San Francisco sheriff’s deputy is facing a felony charge for possession of an assault weapon after being accused of carrying a modified semi-automatic pistol into City Hall, prosecutors announced Tuesday.

Deputy Jalen Rosales, a three-year veteran of the San Francisco Sheriff’s Department, surrendered on a warrant Tuesday after his colleague found the weapon in his backpack last November, authorities said.

“This arrest is a reminder that no one is above the law,” District Attorney Chesa Boudin said in a statement. “My office will continue to hold accountable our law enforcement officers, whom we trust to know and abide by the laws around gun possession.”

The weapon, a Franklin Armory pistol, was found when Rosales left his backpack in a locker room after finishing an overtime shift for an event at City Hall on Nov. 8, 2019, according to the District Attorney’s Office.

“Another deputy, whom Rosales had called and asked to retrieve the backpack for him, found an illegal semiautomatic pistol in Rosales’ backpack,” the office said.

The pistol was a legal weapon that was registered to Rosales, but prosecutors said it had “been modified for use as a semi-automatic weapon with features that make it an illegal assault weapon under California law.”

A spokesperson for Sheriff Paul Miyamoto said the prosecution is the result of a criminal investigation by the Sheriff’s Department.

“We subsequently shared our investigation with the District Attorney’s Office and submitted an application for Deputy Rosales’ arrest warrant,” said spokesperson Nancy Crowley.

Rosales joined the department in April 2017. He is currently on administrative leave and was released from custody after posting bail.

Rosales is scheduled to appear in court for arraignment Aug. 20.

mbarba@sfexaminer.com