Turkish prosecutors on Sunday charged three people with reckless manslaughter over last week’s mining disaster that left 301 dead, AFP reports.

Rescue operations ended on Saturday in the western town of Soma after the bodies of the last two trapped miners were retrieved following the country’s worst ever industrial disaster.

Prosecutors said they have ruled out an electrical fault, which was initially believed to be the cause of the devastating coalmine blast.

A preliminary report into the disaster “suggested the fire could have been caused by coal heating up after coming into contact with air,” prosecutor Bekir Sahiner told journalists, according to AFP.

This could have caused a massive amount of carbon monoxide to fill the mine.

“Twenty-five people have been … detained, including the chairman of the company and three of them have been charged with reckless manslaughter,” Sahiner said.

Six people have since been released and the others were still being questioned.

The Turkish Dogan news agency reported that those charged were plant manager Akin Celik and two mining engineers from mine operator Soma Komur.

Soma Komur has vehemently denied any negligence.

“We have all worked very hard. I have not seen such an incident in 20 years,” Celik said on Friday.

The Soma disaster has sparked a wave of fury against the government, adding to pressure on Prime Minister Recep Tayyip Erdogan ahead of his expected bid for the presidency in August.

Soma was in a virtual lockdown on Sunday after checkpoints were set up on the main roads leading to the town where all demonstrations were banned, AFP said.

Only inspectors and security forces were allowed at the site of the disaster after the rescue teams had left.

On Saturday, at least 36 people, including eight lawyers, were arrested and held in a stadium in Soma after they attempted to make a statement. Some of the lawyers were beaten and injured by police.

Images of police firing tear gas and water cannon at thousands of protesters in Ankara, Istanbul and Izmir have also revived memories of the government’s heavy-handed crackdown against nationwide protests in 2013.

Erdogan had said that mining accidents are in “the nature of the business”, sparking furious accusations of indifference to the victims’ plight.

He has also faced fresh criticism after a video emerged of him apparently shouting an anti-Israel slur at angry protesters during a visit Wednesday to the disaster site.

The disaster adds to the huge political pressure on the prime minister, whose Islamic-rooted party emerged triumphant from March 30 local elections despite a corruption scandal implicating key allies and last year’s mass protests.