California authorities were on a manhunt Thursday for an inmate who walked away from San Quentin State Prison and is believed to have carjacked a vehicle overnight.

Shalom Mendoza, 21, was last seen at the prison before 6 p.m. Wednesday and reported missing after 9:30 p.m. Wednesday. He fled from a work assignment outside the prison walls, spokesman Sgt. Robert Gardea said.

Mendoza is believed to have carjacked a victim in a Home Depot parking lot less than a mile from the prison after 9 p.m., fleeing in a silver Toyota RAV4 with California license plate number 6STZ502, San Rafael police said.

They said a man with a similar physical description to Mendoza, last seen wearing a white T-shirt and khaki or light-yellow pants, motioned to the victim as if he had a weapon or gun under his shirt. He then threatened to kill her and demanded her car keys, but she was not injured.

Mendoza was sentenced to prison a year ago to serve a five-year term for using a deadly weapon during a Los Angeles County carjacking and evading or attempting to evade police while driving recklessly.

He arrived at San Quentin in April and was assigned to a minimum-security inmate work crew in May, Gardea said. Mendoza lived in a dormitory behind a security fence, but he was with a work crew outside the prison fence when he walked away, he said.

San Quentin is California's oldest prison, opening in 1852. It houses more than 4,100 inmates, including those on death row.

Walk-aways by inmates outside of secure areas are not unusual, and most are quickly apprehended without incident. Far more uncommon are escapes from walled or fenced prisons, like Michael Garrett's escape in January from the California Institution for Men in Chino, east of Los Angeles.

Officials released new details on that escape Thursday, telling the Associated Press that Garrett went over a non-lethal fence at the prison holding minimum- and medium-security inmates. He was caught without incident the next day.

The department subsequently improved the fence, lighting and alarms and added staff in the area of the escape, spokesman Jeffrey Callison said.

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This story clarifies that Mendoza left from a work assignment, not a dormitory, based on new information from corrections officials.