Federal safety investigators on Friday interviewed two contractors who were inside Minnehaha Academy when a natural gas explosion killed two people and injured nine earlier this week, but said that their efforts to get a closer look at the building’s gas system have been hampered by the structure’s instability.

National Transportation Safety Board (NTSB) Chairman Christopher Hart said the father-son duo from Eagan-based Master Mechanical spoke with authorities on Friday. They were hired by CenterPoint Energy to move a gas meter from inside the building to outside, Hart said at an afternoon news conference.

He did not reveal what the contractors, whose names haven’t been released, told investigators.

Further interviews will be conducted over the weekend with CenterPoint officials, he said, adding that the probe will seek to determine what might have gone wrong in moving the meter, which he described as a “very hazardous operation.”

“We always look into fatigue; we always look into impairment,” said Hart, pointing out that investigators will scrutinize the contractors’ workload in the 72 hours leading up to the explosion.

CenterPoint officials declined to comment on Friday, and a public relations firm handling media for Master Mechanical did not respond to questions about the incident.

Master Mechanical was issued a permit June 7 for “gas piping and hooking up meter” at the address, city records show. The company did projects at the school in 2003 and 2004.

The building’s structural instability and significant water damage from firefighter’s hoses used to battle the blaze have so far prevented investigators from examining the valves up close, Hart said. Minneapolis fire officials have deemed the building unsafe, he said, and an excavator will have to be brought in to clear away some of the debris.

“We need to get inside the building in the basement to get a look at the valves,” Hart told reporters at an afternoon news conference. City police have turned over body camera footage from the first officers on the scene, he said.

Investigators spent much of the day Friday surveying the blast scene at ground level and from the top of a fire truck ladder, Hart said. The probe is expected to last about a week. A final report may not be released for several months and could take more than a year.

A memorial service for John F. Carlson, 82, a part-time custodian and one of two staff members killed in the blast, will take place at 6 p.m. Sunday at the lower and middle school chapel, 4200 West River Pkwy. in Minneapolis. His family will receive visitors before the service beginning at 4 p.m.

Services for receptionist Ruth Berg, 47, have not been announced. Autopsies showed that both died from “multiple blunt force injuries sustained in a building collapse,” the Hennepin County Medical Examiner’s office said Friday.

In addition to the deaths of Carlson and Berg, a full-time custodian, Bryan Duffey, was also in the area when the blast occurred and was in critical but stable condition.

Duffey, who has been with the school for a year and also is an assistant boys soccer coach, is being treated at Hennepin County Medical Center. One other person remained at HCMC in satisfactory condition.

School officials said the start date for the damaged upper school will be pushed back from Aug. 23 to Sept. 5 while administrators search for a suitable facility for its older students.

Crisis counselors were at the lower and middle schools on Friday and will be available again Monday and as needed the rest of the week. The Christian school had a combined enrollment of 825 students in 2015-16.