The U.S. Department of Education said on Friday that it had opened an investigation into the Oklahoma school district where a 16-year-old student, Nex Benedict, died a day after an altercation inside a high school bathroom.

The department said in a letter on Friday that it was investigating whether Owasso Public Schools, outside Tulsa, had “failed to appropriately respond to alleged harassment of students” in violation of federal law, including Title IX. It said the investigation was in response to a complaint brought by the Human Rights Campaign, an L.G.B.T.Q. advocacy group.

The death of Nex, an Owasso High School sophomore who was nonbinary, drew national attention after gay and transgender rights groups said Nex had been bullied at school because of their gender identity. Nex used they and them pronouns as well as he and him pronouns, friends said.

After the altercation, Nex spoke to a police officer at a local hospital and, according to a video of the interview released by the Owasso Police Department, described pouring water on three girls who had been picking on Nex and Nex’s friends for the way they dressed. The girls then attacked and fought with Nex, who told the police officer that they fell to the ground and “blacked out” at one point.

The next day, Nex’s grandmother and guardian called for an ambulance to rush Nex back to the hospital, where they were pronounced dead.

The cause of Nex’s death remains under investigation by the state medical examiner. The Police Department said in a statement last month that the death was not the result of trauma, but has not elaborated.

Nex’s death brought scrutiny to Oklahoma’s restrictive laws and policies for L.G.B.T.Q. students and to the bullying that family members and friends said Nex had suffered at school.

Karen E. Mines, an acting regional director with the Education Department’s Office of Civil Rights, said in the letter that the opening of an investigation “in no way implies that O.C.R. has made a determination on the merits of the complaint.”

In a statement, the school district said that it was “committed to cooperating with federal officials” and that it “believes the complaint submitted by H.R.C. is not supported by the facts and is without merit.”

The Human Rights Campaign’s president, Kelley Robinson, said, “We need them to act urgently so there can be justice for Nex, and so that all students at Owasso High School and every school in Oklahoma can be safe from bullying, harassment and discrimination.”

At a vigil for Nex last month, Robin Ingersoll, a 16-year-old sophomore and friend of Nex at Owasso High School, said that Nex identified as transgender and that L.G.B.T.Q. students had struggled to find acceptance in their corner of Oklahoma.

“In Owasso, it’s worse than the bullying,” Robin said. “We could all learn more acceptance of others, and be better so something like this doesn’t happen again. We could all grow for Nex.”

Ben Fenwick contributed reporting from Owasso, Okla.

The New York Times