(Reuters) - U.S. safety investigators on Friday began their weeklong inquiry into a deadly explosion at a Christian private school in Minneapolis, including examining whether workers moving a gas meter could have caused the blast.

FILE PHOTO Emergency personnel put water on the scene of school building explosion and collapse at Minnehaha Academy in Minneapolis, Minnesota, U.S., August 2, 2017. REUTERS/Adam Bettcher

National Transportation Safety Board investigators, who took charge of the investigation Friday, planned to speak to contractors, agency spokesman Terry Williams said.

The NTSB, which investigates accidents involving pipelines, previously said it would examine a wide range of potential factors, including worker training. It planned to hold a news briefing Friday afternoon.

The Wednesday morning explosion at Minnehaha Academy caused part of the school building to collapse, killing receptionist Ruth Berg, 47, and custodian John Carlson, 82. Nine people were injured.

Local media reported Berg was scheduled to be married in September. Carlson, who graduated from the school in 1953, was not scheduled to work the day of the blast until 3 p.m. but went in early because he had a doctor’s appointment, according to KARE-TV.

Master Mechanical Inc was issued a permit for gas piping and hooking up a meter in June, according to city documents. The company had no immediate comment on Friday but said in a statement to local media on Thursday that its employees were among the injured.

The Minnesota Occupational Safety and Health Administration cited Master Mechanical in 2014 for two violations, including failing to develop a training program for employees exposed to hazardous substances, agency documents show.

Jenny O’Brien, spokeswoman for the state agency, said such violations were considered routine.

On Friday, Bryan Duffey, an assistant soccer coach at the school, remained in critical but stable condition at the Hennepin County Medical Center, the hospital said. One other person was listed as satisfactory a day earlier, but no information was available on Friday.

Minnehaha Academy was founded in 1913 and teaches more than 800 students from pre-kindergarten through 12th grade on two campuses, according to its website.

School was not in session on Wednesday due to the summer break.