A 23-year-old woman mysteriously died during a flight from London to the U.S. on Monday night, The Guardian reported.

The unnamed passenger boarded United Airlines flight 115 from Heathrow Airport headed for Newark Liberty International Airport in her native New Jersey.

About an hour before landing, flight attendants found the woman unconscious and a doctor onboard tried to help her, officials from the Port Authority of New York and New Jersey said. But the woman, who was traveling alone, did not regain consciousness before the plane landed at around 9:45 p.m.

A medical examiner arrived by 11 p.m. and the body was removed, Port Authority police spokesman Joe Pentangelo told The Guardian. The body was not quarantined.

"We offer our condolences to the passenger's family," said Charles Hobart, spokesperson for United Airlines.

An investigation has been launched into the woman's death, but she apparently had a "pre-existing medical condition," Pentangelo said. No specifics about her condition or any other details were revealed Tuesday.