Three people were charged with second degree murder while a fourth faces involuntary manslaughter in connection with a fatal explosion at the Oxford Center medical facility in Troy that killed a 5-year-old boy, Attorney General Dana Nessel said Tuesday at a virtual news conference.

The victim, Thomas Cooper of Royal Oak, was killed on Jan. 31 while undergoing treatment in a hyperbaric chamber when the device exploded. Thomas’ mother was standing near the hyperbaric chamber at the time of the explosion, and her arms were injured.

The Brighton-based Oxford Center’s CEO Tamela Peterson, primary manager Gary Marken and safety director Jeffrey Mosteller all face charges of second-degree murder and involuntary manslaughter, Nessel said. Aleta Moffitt, the operator of the hyperbaric chamber when it exploded, was also charged with involuntary manslaughter and intentionally placing false information on a medical record.

A second-degree murder conviction could result in up to life in prison, while an involuntary manslaughter conviction could result in up to 15 years behind bars. A jury will be able to decide which charge fits the conduct of the people charged, Nessel explained. Her office is confident that second-degree murder charges are appropriate.

“To prove this charge, a prosecutor must prove that the defendant caused the death, that the death was not justified or excused, and that the defendant knowingly created a very high risk of death or great bodily harm, knowing that death or harm would be the likely result of their actions,” Nessel said.

The four Oxford Center employees are expected to be arraigned in a Troy district court Tuesday afternoon. They were arrested by Troy police on Monday and are currently housed at the Troy Police Lockup facility.

An investigation revealed that safety was among the lowest considerations of several key decision makers at the Oxford Center, Nessel said.

“Due to many failures by men and women who would call themselves medical professionals and wanton or willful disregard for the likelihood that their actions would cause the death of a patient, 5-year-old Thomas Cooper was killed when a fire erupted inside the pressurized, pure oxygen environment inside the chamber,” Nessel said. “The Oxford Center routinely operated sensitive and lethally dangerous hyperbaric chambers beyond their expected service lifetime and in complete disregard of vital safety measures and practices considered essential by medical and technical professionals.”

Marken’s attorney Raymond Cassar said Tuesday he hasn’t seen the police reports yet but said the charges have come as a “huge shock” to Marken and his family.

“Gary was not involved in the overseeing of these hyperbaric chambers, so we’re concerned as to why they would charge him with this, but obviously we have to see the police reports,” Cassar said.

Neither charge is appropriate, said Cassar, who added that the second degree murder charge in particular doesn’t make sense. The explosion was an accident, not an intentional act, he said.

“This was, in fact, a tragic accident, and our thoughts and our prayers really do go out to the family of this little boy,” Cassar said. “We realize that everyone wants answers as to how this happened, including us. We want to see what these experts have come up with, but we do not have those reports at this point in time.”

Hyperbaric chamber safety standards allegedly disregarded by the Oxford Center on the day Cooper died include daily maintenance checks, annual safety inspections and the use of a grounding strap for patients, according to Nessel. There was also no medical doctor or safety supervisor on site when Cooper was undergoing treatment, and his treatment was not performed by a licensed technician, the attorney general said.

“Gary was not involved in the overseeing of these hyperbaric chambers, so we’re concerned as to why they would charge him with this, but obviously we have to see the police reports,” Cassar said.

Neither charge is appropriate, said Cassar, who added that the second degree murder charge in particular doesn’t make sense. The explosion was an accident, not an intentional act, he said.

“This was, in fact, a tragic accident, and our thoughts and our prayers really do go out to the family of this little boy,” Cassar said. “We realize that everyone wants answers as to how this happened, including us. We want to see what these experts have come up with, but we do not have those reports at this point in time.”

Hyperbaric chamber safety standards allegedly disregarded by the Oxford Center on the day Cooper died include daily maintenance checks, annual safety inspections and the use of a grounding strap for patients, according to Nessel. There was also no medical doctor or safety supervisor on site when Cooper was undergoing treatment, and his treatment was not performed by a licensed technician, the attorney general said.

Nessel said the Oxford Center operated the hyperbaric chambers beyond the manufacturer’s intended term of use on children to provide “unaccredited and debunked, so-called treatments.”

Authorities said they do not know for what the boy was being treated in the hyperbaric chamber, a pressurized medical device used to provide oxygen therapy to patients. Among the conditions for which the FDA has approved such chambers for use are burns, carbon monoxide poisoning, decompression sickness and gas gangrene. The Oxford Center advertises hyperbaric oxygen treatment for over 90 different conditions, including attention deficit disorder and traumatic brain injuries.

“To make money as a business, the Oxford center operated these machines, and offered unfounded treatments to patients when medical science contradicted these uses of their services,” Nessels said.

Last month, Cooper’s family said it retained Southfield-based Fieger Law to represent them.