The 12-foot sinkhole appeared near a connection between a pipe that was replaced a year ago and pipe that is decades old, officials said.
The pipe was visible in photos from the scene, which showed two toppled, submerged cars in a massive hole.
The Bexar County Sheriff’s Office identified the victim as a reserve sheriff’s deputy, Dora Linda Nishihara.
Nishihara was a part-time deputy at the county courthouse, the sheriff’s office said, and officials said she was in uniform at the time of her death.
Her car was upside down, almost completely submerged and filled with gravel, dirt and other debris by the time emergency officials arrived at the scene, Fire Chief Charles Hood said at a news conference.
“Our thoughts and prayers are with her friends and family,” the sheriff’s office said on Twitter.
The fire department brought in a 100-ton crane to help lift the vehicles out of the expanding sinkhole.
At least one had been removed by late Monday afternoon.
Working against firefighters: rains that caused a flash flood warning for most of San Antonio, sewage oozing into the hole from the broken pipe and trains that threatened to destabilize everything whenever they rumbled by.
The sinkhole “was a lot bigger than last night,” Robert Puente, the chief executive of the San Antonio Water System, said at the Monday morning news conference. “Every time the train [went] by, we saw pieces of earth falling in.”
This post has been updated.