An OSHA official was conducting an investigation at the Packaging Corporation of America mill in Bradley, Wis., after three men died and one was hospitalized in a blast there Tuesday, the Milwaukee Journal-Sentinel reported.

Tim Brown, acting director of the regional OSHA office said it could be weeks before officials say why a storage tank exploded.

The plant has received three OSHA violation notices in the past five years, one of which was found to be serious, records indicate.