It happened just before midnight on Interstate 10 eastbound near mile marker 492 in Gillespie County, northwest of San Antonio.
Officials said the Pasadena officers were heading on a hunting trip when they stopped on the side of the road to help some stranded motorists.
According to Vance Mitchell, a spokesperson for the Pasadena Police Department, a trailer had come unhitched from another vehicle. Several cars were pulled over with the trailer trying to get it reconnected, and the officers stopped to help, pulling in front of the trailer and other cars.
An 18-wheeler was driving by and swerved to avoid something in the roadway, investigators said. The driver overcorrected and drove into the vehicles on the side of the road.
Patrol Officer Larry Candelari, 49, was killed, officials said. Another officer, who's been identified as 49-year-old Officer Mike Huffman, was taken by LifeFlight to a hospital in San Antonio. Officials say that injured officer lost a leg.
The Texas Department of Public Safety is handling this investigation.
It's unclear how many others were injured, but DPS officials said there were no other fatalities.
Authorities say Officer Candelari was a "seasoned veteran" but did not know exactly how many years he'd been with the Pasadena Police Department.
We're following this story and will bring you the latest developments on Eyewitness News and abc13.com.
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