Another federal investigative agency has joined crews at the site of a deadly natural gas well explosion in eastern Oklahoma to learn more about what caused the accident.

The U.S. Chemical Safety Board (CSB), an independent, nonregulatory agency that’s typically involved in larger industrial incidents, sent two investigators to Red Mountain Energy LLC’s Pryor Trust 1H-9 well near Quinton on Jan. 24. An explosion on Jan. 22 at the site, about 100 miles southeast of Tulsa, killed five workers on a Patterson-UTI Energy Inc. rig that was destroyed in the blast. .

The U.S. Occupational Safety and Health Administration (OSHA) is the lead investigative agency. An OSHA spokesperson said it’s unclear how long the investigation will take. The Oklahoma Corporation Commission (OCC) is also exploring what kind of environmental damage was caused by the incident.

In an initial report drawn up a day after the explosion, the OCC said the rig “was on fire from uncontrolled gas release.” The report said an employee attempted to close the blind rams, or the steel components on the blowout preventer that act to seal the system.

Emergency management officials indicated shortly after the incident occured that some crucial equipment was damaged in the blast. OCC spokesman Matt Skinner said periodic updates are expected as OCC personnel add to the initial incident report.

Contaminated water from diesel fuel and drilling fluids migrated off location, the OCC report confirmed. Red Mountain requested plugging instructions to control the wellbore “until future work can proceed,” according to the report. OCC advised on Jan. 23 that the company should kill the well with heavy drilling mud and plug it with cement.

The operator has also been advised to remove any contaminants from the location and take soil samples from the affected area for testing.