ROSELAND, N.J. — A hospital failed to adequately monitor and treat a New Jersey high school principal as his oxygen level plummeted during a bone-marrow donation procedure in February, sending him into a coma and leading to his death several weeks later, a lawsuit filed Monday by his fiancée alleges.

Westfield High School Principal Derrick Nelson died in April after lapsing into a coma during the procedure at Hackensack University Medical Center, which Sheronda Braker named in her suit.

Nelson, who was 44 and had a 5-year-old daughter with Braker, didn't know the French teenager to whom he planned to donate the bone marrow. They were connected through Be the Match, a worldwide bone marrow registry network.

Westfield High School principal Dr. Derrick Nelson died after attempting to donate bone marrow to a 14-year-old student in France. Westfield Public Schools via AP

Due to privacy obligations, it isn't known if the teen received any bone marrow from Nelson.

In an emailed statement, a spokeswoman for Hackensack Meridian Health said Nelson "leaves a remarkable legacy as an educator and veteran. We are unable to say more at this time due to the litigation process; however, we have been in communication with the family through their legal representation."

Nelson also served as an officer in the Army Reserve for more than 20 years and had recently re-enlisted. He was supposed to marry Braker on June 29. On Monday, Braker said his tuxedo remains in a box in her house.

"Derrick was an amazing human being; very generous, loving, giving of himself," Braker said. "He was the type of person that just wanted to make a difference."

Before the procedure in February, Nelson told the school's newspaper that he had several health issues that complicated his planned donation. His sleep apnea prevented doctors from using general anesthesia, and they instead were to harvest stem cells intravenously.

However, at his final physical exam on Jan. 21, Nelson was asked if he had sickle cell anemia, a blood disorder. "I said well I don't have sickle cell, but I have the sickle cell trait," Nelson told the newspaper. He said the doctors told him that meant they couldn't harvest the stem cells.

They ultimately decided to do the bone marrow surgery under a local anesthetic, he said.

The lawsuit alleges the hospital was negligent because staff continued to administer anesthesia while Nelson's oxygen level decreased and failed to supply him with additional oxygen.

Bone marrow donation is considered a low risk procedure. About 2.4% of donors experience a serious complication due to anesthesia or damage to bone, nerve, or muscle in their hip region, according to the National Marrow Donor Program's website.

"Dr. Nelson lived and died as a hero, and now his family has filed this lawsuit in an effort to seek justice," David Mazie, an attorney representing the family, said Monday.