Thomas Cooper from Royal Oak, Michigan, was pronounced dead at the scene. His mother was standing next to the chamber and suffered injuries to her arms when it exploded January 31 at the Oxford Centre in Troy, suburban Detroit.

Online court records show the centre’s founder and chief executive, Tamela Peterson is charged with second-degree murder. The other defendants are Gary Marken, 65, and Gary Mosteller, 64, charged with second-degree murder and involuntary manslaughter; and Aleta Moffitt, 60, charged with involuntary manslaughter and intentionally placing false medical information on a medical records chart.

Arraignments were scheduled on Tuesday afternoon (Wednesday AEDT) in Troy District Court, Lieutenant Ben Hancock said. The case is being prosecuted by Michigan Attorney General Dana Nessel and details were expected to be released during a late morning news conference.

Raymond Cassar, Marken’s attorney, said the second-degree murder charge comes as “a total shock” to him and his client.

“For fairness, he is presumed innocent,” Cassar said.

“This was a tragic accident and our thoughts and our prayers go out to the family of this little boy.

“I want to remind everyone that this was an accident, not an intentional act.

“We’re going to have to leave this up to the experts to find out what was the cause of this.”

Moffitt’s lawyer, Ellen Michaels, declined to comment before Tuesday’s arraignment. The Associated Press left a telephone message Tuesday morning seeking comment from Peterson’s attorney. An attorney was not listed for Mosteller.

A voicemail was left seeking comment from an attorney representing the Oxford Center. The AP also left a message seeking comment from the center.

The Oxford Centre had said in an email following the explosion that a fire started inside the hyperbaric oxygen chamber.

“The safety and wellbeing of the children we serve is our highest priority,” the centre said.

“Nothing like this has happened in our more than 15 years of providing this type of therapy. We do not know why or how this happened and will participate in all of the investigations that now need to take place.”

Hyperbaric therapy increases delivers pure oxygen to a person’s body inside the pressurised chamber. That’s up to five times the amount of oxygen in a normal room, Troy Fire Lieutenant Keith Young said following the explosion.

“The presence of such a high amount of oxygen in a pressurised environment can make it extremely combustible,” Young said.

“We did some initial investigation. This is very uncommon, so we’re not sure what led up to it.”