Speed is believed to be what caused a driver to lose control of her SUV on an elevated access road, sending the vehicle plunging down about 30 feet onto Interstate 10.
The crash happened around 3:15 a.m. Friday on the eastbound access road near the Wurzbach Road exit.
San Antonio police said the driver was in a turn-around lane but failed to make a turn, instead continuing to go straight ahead.
Her Chevy Trailblazer then hit a curb, went airborne, scraped along a concrete railing and finally landed on its roof on the highway about 30 feet below.
The driver and a passenger, both women in their early 20s, were trapped inside the SUV.
"They were both ... smashed together inside the vehicle,” said Sgt. Scott Foulke. "The fire department was here first. They reported to us that neither was wearing a seatbelt. They were both trapped inside the metal. They were cut out by the fire department."
The passenger was pronounced dead at the scene, while the driver died later at University Hospital.
As of 11 a.m., the medical examiner’s office was still working to identify them.
Foulke said accident investigators are not sure why the driver missed the turn.
"I don't know if she knew she was in the turn-around lane or not, but she was,” he said. “And speed is going to be a factor in this."
Foulke said investigators were also trying to determine if alcohol may have been involved.
The crash left a section of the access road railing with some visible damage.
A worker with the Texas Department of Transportation showed up at the scene later in the morning and placed orange cones around the damaged area.
During the investigation, police shut down several on-ramps and a whole lane of traffic on I-10 in the area near the wreckage.
They reopened everything about 2 hours later.