US federal officials are opening an investigation into the Oklahoma school district where a non-binary high school pupil died following a fight last month, according to a letter sent by the US department of education.

Nex Benedict died a day after the fight inside a girls’ toilet at Owasso High School. Family members say the 16-year-old had been bullied.

According to relatives, Nex identified as non-binary and used they/them pronouns.

The letter revealing the investigation was addressed to the Human Rights Campaign, which had asked the the department to look into Owasso Public Schools and “its failure to respond appropriately to sex-based harassment that may have contributed to the tragic death”.

Police in the Tulsa suburb have not released a cause of death but have previously said that the pupil did not die as a result of injuries from the fight, which took place on February 8.

The department’s Office For Civil Rights stated it would investigate whether the district “failed to appropriately respond to alleged harassment”, according to the letter, which makes no reference to Nex.

Owasso Public Schools confirmed in a statement that the district received notice of the investigation and called the allegations unsupported and without merit.

“The district is committed to co-operating with federal officials,” the statement said.

Neither police nor school officials have said what led to the fight. But Nex’s family has said there had been harassment because of the teenager’s non-binary identity.

Human Rights Campaign president Kelley Robinson said: “We appreciate the Department of Education responding to our complaint and opening an investigation – 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.

Nex walked out of the toilet after the February 7 fight but was taken to a hospital by their family and sent home that night.

Paramedics were dispatched to the home the following day for a medical emergency and took Nex to a hospital emergency room, where they later died, police said.

Nex’s mother, Sue Benedict, has said the teenager suffered bruises all over their face and eyes in the fight involving a transgender pupil and three older girls.

The school district has said the students were in the toilet for less than two minutes before the fight was broken up by other students and a staff member.