COLUMBUS, Ohio (AP) — A critical care doctor was arrested and charged with murder Wednesday in the deaths of 25 hospital patients authorities say were deliberately given overdoses of painkillers.

The charges against Dr. William Husel, 43, form one of the biggest murder cases ever brought against a health care professional in the U.S.

He pleaded not guilty to 25 counts of murder, and a judge set bail at $1 million.

Husel was fired from the Columbus-area Mount Carmel Health System in December and stripped of his medical license after the allegations against him began to surface. An internal hospital investigation found he had ordered potentially fatal drug doses for dozens of patients over his five years at the hospital.

A lawyer for Husel has said he did not intend to kill anyone.

The motive remains unclear. Though many of the patients were seriously ill, hospital officials said some might have improved with treatment, and police Sgt. Terry McConnell said none of the families who talked with police believed that what happened was “mercy treatment.”

Franklin County prosecutor Ron O’Brien said there are no plans to charge other staff members.

More than two dozen wrongful death lawsuits have been filed against the doctor and the hospital system. Mount Carmel has publicly apologized and settled some of the cases for hundreds of thousands of dollars.

Mount Carmel has said it should have investigated and taken action against Husel more quickly. It said that the doctor wasn’t removed from patient care until four weeks after concerns about him were raised last fall, and that three patients died during that gap after receiving excessive doses he ordered.

Hospital officials have said all employees who had a role in administering medication to the victims have been removed from patient care as a precaution.

All told, 48 nurses and pharmacists were reported to their respective professional boards. Thirty of those employees were put on leave, and 18 no longer work there, including some who left years ago, officials said.

In one of the biggest such cases on record, Donald Harvey, a former nurse’s aide dubbed the Angel of Death, confessed in 1987 to killing 37 people, most of them hospital patients, over the span of two decades in Ohio and Kentucky.