The Tulsa County district attorney said on Thursday that no charges would be brought in the fight at an Oklahoma high school last month that involved a nonbinary student, Nex Benedict, who died by suicide the next day.

The district attorney, Stephen A. Kunzweiler, announced his decision in a three-page letter that gave the fullest official account yet of the Feb. 7 fight at Owasso High School, the student’s death and the findings of a weekslong police investigation.

“From all of the evidence gathered, this fight was an instance of mutual combat,” Mr. Kunzweiler wrote. “I do not have a reasonable belief that the State of Oklahoma could sustain its burden of proof beyond a reasonable doubt if charges were presented for prosecution.”

The fight, which took place in a girls’ bathroom, gained national attention after Nex’s death drew outrage from gay and transgender rights groups. They connected the death to the physical altercation and to what family members said was bullying at school.

Earlier this month, the state medical examiner released the results of its examination and found that Nex had died from “combined toxicity” of diphenhydramine, an antihistamine commonly used for allergies, and fluoxetine, a drug often used to treat depression. The autopsy did not show evidence of any internal injury from the fight, Mr. Kunzweiler wrote. The medical examiner’s office listed the manner of death as suicide.

“An important part of the Owasso Police Department’s investigation was the discovery of some brief notes, written by Benedict, which appeared to be related to the suicide,” the district attorney wrote.