HARBIN, July 21  -- A coal mine that covered up a fatal roof collapse for more than two weeks was being operated illegally, authorities in northeast China's Heilongjiang Province said on Monday.

Seven miners died and one was seriously injured when an accident happened in a pit of Xingcheng Coal Mine, formerly known as Yixing, in Hegang City earlier this month, the city government said in a press release.

It said Yixing was closed in December 2009 and merged into Xingcheng Coal Mine. The pit, however, had not been authorized to resume production when the accident happened.

Police said 20 miners were working in the pit when the roof collapsed at around 6:30 p.m. on July 5.

Instead of reporting the accident to authorities, management of the mine organized their own rescue work. By 9 a.m. the next day, four dead bodies were retrieved and four others were sent to hospital with injuries, three of whom failed to respond to emergency treatment and died.

The mine management secretly cremated the bodies and compensated families of the victims.

Police have detained the mine owner and managers in connection with the accident.