At least four miners were killed after a structural collapse at a coal mine in northern Russia, and the rescue operation was further complicated by a second explosion when emergency workers were trying to reach dozens of miners trapped underground, officials and media reports said Friday.

About 110 coal miners were underground at the “Severnaya” coal mine — operated by the Vorkutaugol company in the northern region of Komi — when a seismic jolt caused part of the structure to collapse on Thursday, the regional branch of the Emergency Situations Ministry said in a statement. Shortly before midnight, 80 of the miners had been rescued, the ministry said. Eight of them had suffered injuries.

Rescuers have also recovered the bodies of four more miners, while 26 others were listed as missing as of Friday afternoon, according to Russian media reports.

About an hour after the initial jolt, when attempts to evacuate workers were already underway, the mine suffered a second “explosion,” Vorkutaugol technical director Igor Paykin said, the Interfax news agency reported Friday.

The second blast caused additional structural collapses in the mine, stoked a fire, and caused clouds of smoke to fill the mine shaft, Paykin was quoted as saying.

“It appears impossible to extinguish it [the fire] through ordinary methods,” he said, Interfax reported. “We will look into the option of temporarily isolating the combustion section.”

Emergency Situations Minister Vladimir Puchkov, who was in Vorkuta on Friday to direct the rescue operation, said his agency dispatched an Ilyushin-76 strategic airlifter to bring an additional 70 rescuers to the site to “organize continuous shift work by mountain-rescue teams,” Interfax reported.

The primary version of the event considers human factors the root cause, an unidentified source in the region’s technical inspection services told the Interfax news agency. The source added that the reasons for the tragedy could be complex, including both human and natural factors.

Meanwhile, according to Vorkutaugol, the incident was caused by a methane explosion, the TASS news agency reported, citing the company’s spokeswoman Tatyana Bushkova. Russia’s Investigative Committee has opened a criminal case over the violation of safety rules during operations at the mine.

Russian coal mines have suffered multiple accidents in recent decades and are considered among the most dangerous in the world. Part of the danger stems from the lack of modern safety features at many of the facilities.