Federal officials investigating the deadly West Texas collision between at Union Pacific train and a tractor-trailer hauling a wind turbine base are trying to determine why the tractor-trailer was stopped on the tracks.

Authorities say two Union Pacific employees were killed and three people were injured when a freight train collided with a tractor-trailer and derailed in a small West Texas city.

Officials in Pecos, Texas, are picking up the pieces after a train derailed, crashed into a building, killed one person and injured four others on Wednesday.

While residents smelled rotten eggs, the state’s environmental agency said it has not detected any air pollution.

Experts warn that more blowouts should be expected unless oil and gas companies change their methods.

Injecting saltwater back into the ground “is likely contributing to recent seismic activity,” the Railroad Commission of Texas has said.

The 5.2 magnitude earthquake is tied for the fourth strongest in Texas history. It occurred in an area where oilfield companies have long been injecting wastewater from fracking underground.

An exclusive Inside Climate News analysis found that companies have spilled nearly 150 million gallons of toxic, highly saline wastewater in Texas over the last decade.

Operators pump a sea of “produced water” underground for disposal. Intensifying tremors raise fears that the deep toxic waste pits could intermingle with water used for farming and drinking.

The quake was the third largest in Texas history, according to the U.S. Geological Survey. An increase in seismic activity in Texas has been linked to injecting fracking wastewater underground.