The Police Department is investigating the death of an 18-year-old woman in a cell at a Bronx police precinct early Saturday, the authorities said on Monday.

The woman, Saniyah Cheatham, who had been arrested on Friday, was found unconscious just after 12:30 a.m. at the station house of the 41st Police Precinct in the Longwood neighborhood of the Bronx, the police said. The police did not say why Ms. Cheatham was arrested or whether she was charged with a crime.

How Ms. Cheatham died is unclear. Julie Bolcer, a spokeswoman for the city medical examiner, said an autopsy was performed on Sunday, and a cause of death is pending.

But according to two law enforcement officials with knowledge of the matter who were not authorized to speak about the case, Ms. Cheatham was inside her cell when, around 12:15 a.m., she hanged herself with her sweater.

The department’s Force Investigation Division is investigating the incident, the police said.

Ms. Cheatham’s friends were stunned, saying that she appeared to be in good spirits on Friday morning before attending a Fourth of July family barbecue in the Bronx.

Her close friend, Ember Baez, 20, said she texted with Ms. Cheatham on July 4, and nothing seemed amiss.

“Everyone who has spoken to her recently knows she’s been happy,” Ms. Baez said. “So when I heard this happened, I called her multiple times to see if it was real, and her phone just kept going straight to voice mail.”

“It makes no sense,” she added.

After Ms. Cheatham’s arrest on Friday, officers brought her to the 41st Precinct, the police said.

Just after midnight, officers led Ms. Cheatham into a cell, according to one of the officials. Within two minutes, she had taken off her sweater and used it to hang herself, the official said. It was unclear on Monday whether Ms. Cheatham was in the cell with other detainees, or whether anyone else witnessed her actions or the actions of the officers.

At 12:40 a.m., the officers, who are required to perform routine cell checks once every half-hour, discovered Ms. Cheatham, according to the official and the police.

The officers performed cardiopulmonary resuscitation while waiting for an ambulance, the police said. Ms. Cheatham was taken to NYC Health and Hospitals/Lincoln, where she was pronounced dead.

If you are having thoughts of suicide, call or text 988 to reach the 988 Suicide and Crisis Lifeline or go to SpeakingOfSuicide.com/resources for a list of additional resources.