Officers responded to the southbound lanes of Interstate 35 near CM Allen Parkway around 2:45 a.m.
A vehicle that was possibly stopped in the road or going slow in the right lane was hit from behind by a 2007 Chevrolet dump truck.
The driver of the vehicle was killed instantly and a passenger in the vehicle was pronounced dead after being taken to Central Texas Medical Center in San Marcos.
The driver of the dump truck, 20-year-old Isaac Fettig, was taken to Seton Medical Center Hays in Kyle with minor injuries.
The southbound lanes of the highway were shut down for several hours while police investigated.