The Texas utility workers who accidentally hit a gas line, causing a deadly explosion, followed proper procedure to locate the line before drilling, an official said Tuesday.

Fred Haag, chief operating officer of Oklahoma-based C&H Power Line Construction Services, said the crew had a survey map of the gas lines and also made precautionary calls to verify the line location.

The 23 member crew accidentally struck a natural gas line at a site in rural Johnson County, Monday. One worker was killed in the explosion and seven others were sent to the hospital.

The crew had been working in the area for several months. On the day of the explosion, they were drilling a hole for a utility pole.

Investigators want to know if the gas line was marked before the digging began. Workers will repair the ruptured line once the investigation is complete.