Marilyn Hartman wouldn't stand out to most folks crisscrossing through O'Hare International Airport.
With short white hair and a thin build, the 66-year-old, occasionally bespectacled Grayslake woman looks like she could be a retired schoolteacher or somebody's grandmother.
What she is, is a convicted criminal -- a repeated stowaway who has frustrated airport officials, police and federal security agents for years.
She was arrested twice at O'Hare last month -- once for taking a flight to London without a ticket, and again days later for being at the airport despite a court-ordered ban.
Hartman's actions are prompting security changes at the airport. The Transportation Security Administration already has altered passenger checkpoints and queue configurations to prevent people from circumventing TSA agents checking documents, agency spokesman Mike McCarthy said, and more changes could be coming to O'Hare and other airports across the country.
"We take this case very seriously," McCarthy said.
Although authorities don't believe Hartman posed a danger to other people at O'Hare, her actions exposed weaknesses in airport and airline security procedures, aviation security expert Jeff Price said.
"The security risk is that someone on the no-fly or (suspected terrorist) list could board a flight without the TSA knowing about it," said Price, a professor in the aviation and aerospace department at Metropolitan State University of Denver.
Long criminal history
Hartman has been stowing away on airplanes -- or trying to -- for years. She's been arrested many times, often just for being at an airport.
She also has struggled with mental illness and homelessness, according to media reports.
Hartman's encounters with police at airports include:
• A 2010 arrest at a Hawaii airport.
• Six arrests at San Francisco International Airport in early 2014.
• A 2014 arrest for taking a Southwest Airlines flight from San Jose to Los Angeles without a ticket, for which she pleaded no contest to a misdemeanor count of willfully and unlawfully entering the city as a stowaway on an aircraft, according to The Associated Press.
• Four arrests in 2015 at O'Hare and Midway airports.
• A February 2016 arrest at the O'Hare shuttle bus center, which followed an order to stay away from the airport. In that case, she pleaded guilty and was sentenced to six months of house arrest and was placed on two years' mental health probation. At the time, she was living at a Chicago-area mental health facility.
Hartman's methods have varied. Once, she was stopped at a gate with a discarded boarding pass, media reports indicate. In another incident, she sneaked past a security screener who was busy checking a family's documents.
Security changes were made at the various airports Hartman visited after her illicit flights and attempted flights, the TSA said. No protocols were changed nationwide, however.
After one of her 2014 arrests, Hartman claimed she had cancer and wanted to go somewhere warm, the San Francisco Chronicle reported. But Hartman later said she didn't have cancer, according to San Francisco magazine.
She's also spun tales of a vast conspiracy that compels her to sneak onto planes, according to the magazine.
New O'Hare arrests
Her two arrests last month at O'Hare prompted the newest TSA security changes.
In the first case, Hartman got through security without a ticket or passport Jan. 14 and, the next day, boarded a British Airways flight to London, Chicago police said.
Hartman got past a TSA checkpoint and went through the airport metal detector screening without being discovered, McCarthy said. He declined to say how Hartman circumvented the checkpoint, where an agent is supposed to confirm people have proper ID and boarding passes.
McCarthy also declined to explain how Hartman boarded the plane without a ticket.
Upon landing at Heathrow Airport, Hartman was detained by British customs officials and returned to Chicago, where she was arrested.
Hartman was charged with felony theft and a misdemeanor count of criminal trespassing. A Cook County judge subsequently ordered Hartman to undergo a mental health evaluation and to stay away from O'Hare and Midway International Airport.
But on Jan. 28, Hartman was arrested again at O'Hare after police responding to a disturbance say they found her inside Terminal 3.
Hartman was charged with misdemeanor criminal trespassing to state land and violating her bail. She's being held without bail at Cook County jail.
Other passengers or visitors to O'Hare were never at risk because of Hartman's actions, said Lauren Huffman, a spokeswoman for the Chicago Department of Aviation, the agency that oversees O'Hare and Midway.
But that doesn't mean her actions aren't cause for worry.
"The fundamental problem with this situation is that she was able to bypass a passport check, a TSA ticket and ID check, and an airline ticket check to be able to board a flight, and she's done this several times," said Price, who wrote a book on aviation security.
Hartman's habitual flying attempts shows profiling passengers based on race or nationality for security risks isn't as effective as watching behavior, Price said.
"To most people she looks harmless," he said. "Few people are going to give her much notice because she doesn't look like what they think terrorists or criminals look like."
That Hartman got on a plane Jan. 15 despite being screened by TSA indicates one important security check was conducted but that "other layers" of airport security failed, Price said.
At least one of those failures occurred at the gate, because people aren't supposed to board planes without valid passes.
The TSA is going to work with airlines to see if procedural changes need to be made "or if this was an isolated incident or lapse," McCarthy said.
"We, along with industry partners ... are currently reviewing ways to keep this from happening again," he said.
McCarthy also said the TSA and industry officials are investigating if "additional physical security measures" at checkpoints could deter would-be stowaways.
He declined to elaborate on what security changes could be coming or how air travelers could be affected, saying such speculation would be premature. The changes already implemented at O'Hare haven't negatively affected airport visitors, he said.