A severely injured man entered a Red Line train early Tuesday and was found later in the South Loop.
Authorities are unsure how or where he was hurt.
Witnesses told investigators the man was already injured when they found him on a train about 1:40 a.m., Chicago police said.
Paramedics took him from the Harrison Street station to Northwestern Memorial Hospital, the Chicago Fire Department said. He was in serious-to-critical condition.
Police said the man declined to file a report with officers.
A CTA employee told ABC7 that the man looked bloody around the mouth, as if he was beaten.
Security video showed the individual enter a station after already suffering visible injuries, the Chicago Transit Authority said in a statement Tuesday afternoon. A CTA spokesperson said they were not sure which station he entered.
There was no sign that he was injured on or near CTA property, according to the statement. The CTA said it would share security video with Chicago police.
Earlier Tuesday, police said they were not investigating the case.
A spokesperson for the Chicago police was not immediately sure Tuesday afternoon if a case had been opened.