A team of investigators from the National Transportation Safety Board will determine the cause of the crash of the Union Pacific and Burlington Northern Santa Fe trains in a rural area southwest of San Antonio.
Track conditions, braking systems, signal operations and how the respective crews responded to the situation will be studied.
A Union Pacific dispatcher had arranged to have the westbound UP train stop while the eastbound Burlington Northern train pulled off onto a siding, according to an investigator.
"Somebody should have stopped and didn't," he said.
Twenty-three of the 74 cars on the UP train derailed. Seventeen Burlington Northern cars derailed, all of them empty.
A tanker car carrying pressurized chlorine broke open in the collision, venting tons of gas into the air. A small amount of ammonium nitrate was also released from three UP cars.
The gas clouds dissipated after drifting up to ten miles to San Antonio's Sea World amusement park, where six people were treated for minor respiratory irritation.
An 85-year-old man and 59-year-old woman died from the effects of breathing chlorine gas. Their bodies were found in their home about a mile from the crash.
Also killed by the gas was a 23-year-old conductor aboard the Union Pacific train.
Two people who lived near the crash remained in critical condition. A San Antonio Fire Department spokesman said more than 40 other people suffered mostly minor respiratory irritation.