Washington ? A Union Pacific freight train crew probably was asleep at the controls, causing a fatal Texas train crash that released deadly chlorine gas, the government said Thursday.

The conductor had been drinking before starting work and the engineer had been on the job for 37 hours out of 55 hours shortly before the accident on June 28, 2004, the National Transportation Safety Board’s staff reported.

Increasingly, investigators are focusing on heavy workloads of train crews as a possible factor in wrecks.

The train went through a caution signal and a stop signal before plowing into a Burlington Northern Santa Fe train that was moving onto a rail siding near Macdona, Texas.

In the accident, a tank car was punctured, sending a cloud of chlorine gas 10 miles. Three people died from chlorine gas, including the train conductor.