While she was on the flight, the airline - British Airways - realised she did not have a ticket. Her arrest last week was at least the 10th time she's been caught trying to slip past airport security without a plane ticket.
Marilyn Hartman, 66, faces a felony theft charge in connection with flying to the United Kingdom on a British Airways jet without a $2,400 ticket and a misdemeanor trespass charge in connection with getting into the airport illegally.
More recently, Hartman spent a year in Cook County Jail after repeated episodes of attempting to board aircraft at O'Hare and Midway worldwide airports, according to CBS Chicago.
She then boarded a flight to the United Kingdom and was detained by British Customs Officials at Heathrow International Airport in London on January 15, police said. When it could be arranged she was returned to Chicago and was arrested at O'Hare on arrival.
Hartman reportedly said she "may have" boarded aircrafts without a ticket eight times, the report said.
"Upon learning of the incident, TSA and its aviation partners took immediate action to review security practices throughout the airport", the agency said Friday.
The Transportation Security Administration is investigating how she managed to sneak past security.
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Marilyn Jean Hartman on Tuesday, Aug. 26, 2014.
"We are working with our law enforcement partners to support a comprehensive and thorough investigation, while continuing to maintain the highest levels of security at O'Hare Airport", Huffman said in a statement.
They said in a statement: "During the initial investigation it was determined that the passenger was screened at the security checkpoint before boarding a flight". It's unclear how she evaded federal authorities. Her movements were caught on video.
A judge ordered so-called "serial stowaway" held on $25,000 bond Saturday after she was arrested for taking yet another unauthorized flight.
Ms Hartman also has a history of attempted escape from mental health facilities.
According to a Chicago Tribune report in March 2016, Hartman pleaded guilty after she trespassed into state land and was arrested at a bus shuttle center at the same Chicago airport.
She also told police that she felt safer inside the airports than living on the streets.