NEW YORK (AP) — Police are investigating the death of an unidentified 53-year-old woman found unconscious on a subway car in the Bronx.

Police say the woman was pronounced dead at the scene aboard a northbound Number 1 train at the Van Cortlandt Park subway station at about 8:45 a.m. Monday. She had no obvious signs of trauma.

The Daily News reports a witness telling an MTA worker the woman had boarded the train on the Upper West Side of Manhattan. The worker says the woman slumped over during the ride, but thought she was just sleeping.

The medical examiner is to determine the exact cause of death.