Four people have been charged in the death of a 5-year-old boy who was “incinerated” inside a pressurized oxygen chamber that exploded at a suburban Detroit medical facility, Michigan’s attorney general said Tuesday.

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 Jan. 31 at the Oxford Center in Troy.

Online court records show the center’s founder and chief executive, Tamela Peterson is charged with second-degree murder. Also charged are facility manager Gary Marken, 65; safety manager Gary Mosteller, 64; and the hyperbaric chamber’s operator that day, Aleta Moffitt, 60.

Marken and Mosteller are charged with second-degree murder and involuntary manslaughter. Moffitt is charged with involuntary manslaughter and intentionally placing false medical information on a medical records chart.

All were arrested Monday pending arraignments Tuesday afternoon in Troy District Court, Attorney General Dana Nessel said at a news conference Tuesday.

Nessel said the defendants unscrupulously put children’s bodies at risk through unaccredited and debunked treatments, simply because it brought cash through the door.

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 Center 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 center 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 pressurized chamber. That’s up to five times the amount of oxygen in a normal room, Troy Fire Lt. Keith Young said following the explosion.

“The presence of such a high amount of oxygen in a pressurized 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.”