A national firefighters union is concerned about a safety investigation into the fire that killed nine Charleston, S.C., firefighters, the Associated Press reports.

The International Association of Firefighters says a state OSHA investigator has never handled a fatal fire. The international union says top city fire officials attended OSHA interviews with firefighters.

OSHA says the investigator knows workplace and fire safety laws.

OSHA also says all firefighters had a chance for private interviews. The city says officials attended interviews to prevent firefighters from having to tell their stories several times.

The nine firefighters were killed in a furniture warehouse fire June 18.