The Saturday raid was carried out over a suspicion of professional negligence resulting in death and injuries -- in addition to those killed, 12 people were injured by the blast, Kyodo News reported.
During the Saturday raid, authorities revealed the heat exchanger that exploded Thursday explosion had not been cleaned since its installation in January 2006. A power generator had also been placed near the heat exchanger, despite the risk of sparks.
The explosion occurred while about 20 workers were cleaning the heat exchanger.
Mitsubishi Materials President Hiroshi Yao has issued an apology for the explosion, but has not stated the cause.
"Although the specific cause of the accident is still unknown, we will take thorough measures to prevent similar incidents," Yao said.
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