OSAKA, Japan (AP) - Four men died in an explosion Thursday at a chemical plant in Osaka, firefighters said.

The blast occurred at around 8:50 a.m. at a fluorine compound production plant operated by Morita Chemical Industries Co. in Osaka's Yodogawa Ward.

Those who died in the blast are Kazuhiro Hayashi, 47, Tomoyuki Kanazawa, 33, Tatsuya Nakatani, 45, and Keisuke Nishida, 30, all of whom were employees at the company, police said.

The explosion occurred when the four workers were cutting away the top of a chemical tank as part of efforts to clean it. The tank stores a toxic chemical material called borontrifluoride.

Authorities are continuing to investigate the matter, they said.

The firefighters said part of the plant's roof was blown off in the blast. One of the four victims was found collapsed on the roof, another on the ground, and the remaining two were found in the plant.

Yasuo Morita, the company's president, said at a press conference, "I feel social responsibility (for the blast). I'm really sorry."

Morita Chemical Industries, an unlisted chemical maker based in Osaka's Chuo Ward, said it produces fluorine compounds for industrial use at the plant.

The chemicals are used in semiconductor production and as electrolytic solutions for lithium batteries, it said.

Usually, fluorine compounds are unlikely to explode, the company said.

Founded in 1935, Morita Chemical Industries produces and sells hydrofluoric acids, fluorine compounds and various other chemicals as well as pharmaceutical products, the company said on its website.

The plant is located in a housing-industrial complex some 1.5 kilometers north of Shin-Osaka Station, the Osaka terminal of the Tokaido Shinkansen bullet train line.