A Chicago Transit Authority worker was being questioned by police Saturday in the death of a man at a Blue Line station in the Loop.

The 54-year-old man was found unconscious about 4 a.m. Saturday on a stairwell landing at the LaSalle Street station in the 100 block of West Congress Parkway, according to Chicago police.

The man, who has not been identified, was taken to Northwestern Memorial Hospital, where he was pronounced dead, police said.

“One person of interest is in police custody and is currently being questioned,” police said.

A law enforcement source told the Sun-Times the person in custody was an employee of the transit agency who was allegedly seen on surveillance video assaulting the man before he was found unconscious.

In an emailed a statement, an agency spokesperson said the “CTA is working closely with Chicago Police regarding an incident last night at the LaSalle Blue Line station involving a CTA employee.”

Area 3 detectives are conducting a death investigation pending an autopsy expected to be conducted Sunday by the Cook County medical examiner’s office to determine the cause and manner of the man’s death.