MINNEAPOLIS >> Investigators from the National Transportation Safety Board arrived at a Minneapolis school today to begin the painstaking task of determining what caused an apparent natural gas explosion that killed two people and injured at least nine others, including one critically.

The explosion at Minnehaha Academy caused part of a school building to collapse Wednesday. The bodies of longtime school receptionist Ruth Berg and custodian John Carlson were found in the rubble.

City fire officials said the collapse was caused by a natural gas explosion in a utility area. Contractors were working in the school at the time, and some witnesses said they were warned of a gas leak moments before the blast. Some first responders also reported smelling natural gas as they pulled people to safety.

NTSB spokesman Eric Weiss said the federal agency is investigating because it has jurisdiction over gas pipelines. Experts said the investigation will take time and include interviewing witnesses, including employees from Master Mechanical Inc.

The company was issued a permit on June 7 for “gas piping and hooking up meter” at the school, according to city records. The permit did not include details on the type of work involved. Master Mechanical said in a statement that its employees were among the injured.

Mark Farley, a Houston-based attorney who specializes in major accident response, said investigators are going to be honing in on the cause of the blast. He said that includes examining physical evidence and interviewing potential witnesses.

Questions will include what type of work was being done, whether the building should have been occupied at the time, if all precautions were taken to make sure it was done safely, and whether the contractors should have isolated the natural gas source before proceeding, Farley said.

“The investigators at this point are going to try to determine what the root cause of the incident was,” he said.

He said at this point, it’s premature to say whether the building should have been cleared beforehand. He said the limited information on the permit suggests the contractors were working directly on the gas lines.

“The question is going to be, was it isolated in a way that they were confident they could do the work without the risk of a leak,” he said. “Obviously something went terribly wrong.”

Tim Fetterly, a Minnesota attorney experienced in cases stemming from fires and explosions, said the investigation will likely take months. The process can be accelerated if a witness knows exactly what went wrong, but investigators still need to look at all of the physical evidence.

He said investigators will likely remove all the gas components from the scene and conduct lab tests.

Fetterly said gas is meant to be transmitted in a closed system, and finding the source of the leak will be key. He said an ignition source is usually not paramount in a case like this because anything from a light switch to a pilot light could create enough of an arc to ignite the released gas.

“Oftentimes because there are so many possibilities like that … the exact ignition source is often not something they are able to determine,” he said.

Master Mechanical has been cited for workplace violations twice in recent years. Jenny O’Brien, a spokeswoman for Minnesota Occupational Safety and Health Administration, said in 2010 the company was fined for a violation related to protecting an employee from falling. In 2014, the company had two paperwork violations related to letting employees know about job hazards.

O’Brien said Minnesota OSHA also had investigators at the site Thursday and would be looking into employee safety and training regulations.

Three people remained hospitalized Thursday, including an assistant soccer coach who was in critical but stable condition, according to Hennepin County Medical Center. Bryan Duffey’s family released a statement Thursday saying he has traumatic injuries that will require more surgery.

Six other patients who were brought to the hospital after Wednesday’s blast have been released.

Minnehaha Academy is a private, Christian school that serves students from pre-kindergarten through 12th grades. The explosion affected only the “upper school,” which houses the high school. The lower and middle school campus is about a mile and a half away.

Aerial video footage of the blast site showed part of a building was ripped apart, with wood splintered and bricks scattered about. Windows in other areas were blown out and shattered.