Safety conditions in Romanian mines came under scrutiny Sunday after the country`s deadliest mining accident in recent years left 12 dead and 13 injured.

`This is too big a tragedy not to wonder about the causes,` said President Traian Basescu, who travelled overnight to the scene.

Eight miners were found dead and another eight wounded when a methane gas explosion occurred Saturday afternoon at the Petrila site in the Jiu valley, Romania`s main mining region.

A second explosion some four hours later killed four rescuers, themselves miners who had gone down the shafts to retrieve the bodies, and injured five.

Immediately afterwards, rescue teams were ordered to return to the surface and all operations were suspended for 48 hours.

`The inquiry will determine if all protective measures were taken when the first miners went down the shafts and if the rescuers` intervention was done following the rules,` Economy Minister Varujan Vosganian said.

Miners and relatives of the victims who had gathered at the entrance to the mine said methane gas levels inside had already exceeded accepted limits on Monday, causing smaller explosions.

But several miners, who refused to be named, said they had been forced by their bosses to continue their work.

Ana Alexa, whose brother-in-law was killed and husband injured in the first blast, said an exposion had occurred on Tuesday but that it was `covered up.`

`If a miner says there was an accident in the mine, his salary is cut. If he asks for better equipment, he`s told to look for another job,` she told Realitatea TV.

`It`s criminal negligence to let these people work when the (accepted) methane gas level has been topped,` the vice-president of the national trade union bloc (BNS), Ovidiu Jurca, added.

`After the first explosion, no other miners should have been allowed to go down the shafts.`

The president of the Oltenia miners` union (CNSLO), Marin Condescu, said mine management probably thought the rescue teams were not in danger.

`An hour after the first explosion, methane gas levels were back to the accepted norm,` he said.

`But there was probably a hidden pocket of gas somewhere,` he told AFP.

Many witnesses have said working conditions in the mine, one of the oldest in Romania, were especially difficult with obsolete equipment and temperatures reaching 50 degrees Celsius (122 Fahrenheit).

Unions have blamed a lack of investment in the mining sector but the economy minister noted that state subsidies were limited by the European Commission.

`In 2008, the national coal company (CNH, which operates Petrila) received about 3.6 million euros, which is the maximum allowed by the commission,` he said.