Mount Carmel Health System, the second-largest health care system in central Ohio, began an internal inquiry earlier this year and discovered one of its doctors had over-prescribing potent doses of painkillers leading to over 25 deaths; shortly after a criminal investigation was launched, reported NBC News.

William Husel, the Ohio doctor at the center of the case, surrendered to Columbus police and was charged Wednesday with 25 counts of murder for ordering excessive doses of the opioid fentanyl for patients between 2015 to 2018.

Court documents show Husel prescribed at least 500 micrograms of fentanyl (100 micrograms of the synthetic opioid is normal for a patient) for at least 25 patients, levels that are considered deadly. “At the 500-microgram level there would be no legitimate medical purpose,” said Franklin County Prosecutor Ron O’Brien, according to the Columbus Dispatch. “The only purpose would be to hasten their deaths.”

Many of the deaths were people who were older and already had severe health issues, but a motive behind why Husel overprescribed dangerous amounts of fentanyl remains a mystery, officials said.

“This is not a murder case,” Husel’s attorney, Richard Blake, told NBC affiliate WCMH. “I can assure you there was never any attempt to euthanize anyone by Husel. At no time did he ever have the intent to euthanize anyone.”

Husel pleaded not guilty and was slapped with a $1 million bond Wedsenday. He has surrendered his passport and wants to clear his name at trial, Blake said.

Husel faces up to 375 years in prison if convicted on all counts. O’Brien said the doses Husel prescribed in the 25 deaths “could not support any legitimate medical purpose.”

Mount Carmel Health System, first broke the news about the case in early January, after prosecutors and police began questioning dozens of witnesses and analyzing medical records.

In one suit, an attorney for the family of Melissa Penix,82, said Penix was given 2,000 micrograms of fentanyl by Husel for stomach pains in November and died five minutes later.

Amy Pfaff’s mother, Beverlee Schirtzinger, 63, had a liver biopsy in October 2017 before her health rapidly deteriorated. Pfaff’s lawyers said Husel ordered 500 micrograms of fentanyl for her.

The State Medical Board of Ohio suspended Husel from the medical system in late Januray, indicating the decision was based on his “failure to meet acceptable standards regarding the selection of drugs, violations of the minimal standards of care and failing to cooperate in a board’s investigation.”

Husel’s wife, Mariah Baird, was also named in one of the suits for administrating 800 micrograms fentanyl to a 65-year-old woman who died in 2015, at the same hospital.

Hospital officials said 30 employees, including pharmacists and nurses, were placed on administrative leave earlier this year following the investigation, while at least 18 others with ties to the case have left. Even the hospital’s chief pharmacy officer, Janet Whittey, recently abandoned her position after being tied to one of the lawsuits.