Five workers who went missing on Monday after a Patterson-UTI Energy Inc. drilling rig in eastern Oklahoma exploded and collapsed are presumed dead, local emergency management officials said on Tuesday.

“At this time, we have moved from a rescue mission to a recovery mission,” said Pittsburg County Sheriff Chris Morris. “Fire rescue personnel have performed primary and secondary searches of the area for those missing without any success.”

Seventeen other workers were safely evacuated. One was airlifted to St. John Medical Center in Tulsa to treat burns. Morris said those presumed dead include Josh Ray, 35, of Fort Worth, TX; Matt Smith, 29, of McAlester, OK; Cody Risk, 26, of Wellington, CO; Parker Waldridge of Crescent, OK, and Roger Cunningham of Seminole, OK.

Ray, Smith and Risk were all employees of Patterson. It’s unclear what company the other two men worked for. Their ages were also not released. Morris said it is most likely that the missing men were working in the doghouse or on the rig floor at the time the accident occured. It remains unclear what exactly happened or what caused the explosion.

“We can’t speculate and we won’t speculate on the investigation at this point, but we will work with the Occupational Safety and Health Administration (OSHA); we have management working with OSHA today to begin the investigation because we want to learn from this,” said Patterson CEO Andy Hendricks, who joined emergency management officials on Tuesday for a press conference at Quinton High School near the scene. “We don’t want this to happen again for anybody in our industry.”

Authorities received a call at 8:45 a.m. CST on Monday and were dispatched to a site west of Quinton, which is about 100 miles southeast of Tulsa. Footage of the aftermath of the explosion showed flames spewing into the air and eventually a collapsed and charred derrick. By Monday night, the fire had been extinguished. Halliburton Co. subsidiary Boots & Coots Services was called to assist in bringing the wellhead fire under control.

Ten fire departments, law enforcement agencies, three medical helicopters, crisis management teams and even the Red Cross, among others, responded to the scene, according to Pittsburg County Emergency Management (PCEM).

“We’re waiting on the scene to be declared safe,” PCEM Director Kevin Enloe said Tuesday. “We’re relying on the resident experts to tell us that they feel comfortable and that we’re not going to have anymore issues with the medical examiner and their team to conduct their follow-up.”

Patterson was working for Oklahoma City-based Red Mountain Energy LLC. The producer had no personnel on site at the time of the incident, said Kevin Say, who co-founded the company in 2013, and joined officials at the press conference on Tuesday.

Oklahoma Corporation Commission (OCC) spokesman Matt Skinner said the company’s affiliate, Red Mountain Operating LLC, was authorized to drill a horizontal natural gas well at the location with a permit for the Mississippian Lime and Hunton formations and the Woodford Shale.

Hendricks, who leads a major North American contract driller with more than 7,000 employees, called the incident a “terrible tragedy and a terrible loss.” He stressed that safety is always the primary concern for his company and joined Say in expressing condolences for the families of the dead, which had already been notified on Tuesday.

“Like you, the public, the media, we want to know what caused this horrible event, but today is not the time for those questions, at least not for me,” Say said. “At this moment, we’re focused on the difficult days ahead of us. Our priority, sincerely, is the people who have been affected and the safety of those who will be called upon to work the well site and investigate.

Skinner said OCC records show the rig was still in the process of drilling the well’s vertical leg when the accident happened. Patterson’s Rig 219 was a large, 1,500 hp APEX model equipped with a hydraulic walking system. Say estimated that the rig was at a depth of about 13,500 feet on the way to a total depth of about 17,000 feet when the incident occured.

Red Mountain was formed in 2013 and has been led by former Chesapeake Energy Corp. employees, including Say. The company was formed to focus on the Midcontinent and Permian Basin regions. In Oklahoma, its exploration efforts are focused on liquids-rich targets with associated natural gas. Say described Monday’s accident as a first for the company and said his staff is “devastated.”

Skinner said because the explosion was a “workplace accident,” OSHA and the Oklahoma Department of Labor would handle the investigation to determine what happened and assess any necessary fines. The OCC is primarily concerned with remediating any environmental damage to land or water.