Nine American Eagle airplanes were grounded Tuesday after a TSA inspector, conducting an overnight security check, used sensitive instrument probes to climb onto the parked aircraft at Chicago's O'Hare Airport, aviation sources tell ABCNews.com.
A TSA official confirmed the incident.
At least forty regional commuter flights were delayed throughout the day, according to American Airlines.
"We think it's an unfortunate situation," American airlines spokesperson Mary Frances Fagen told ABCNews.com.
The TSA agent, as part of spot inspection of aircraft security, climbed onto the parked aircraft using control sensors mounted on the fuselage as handholds, according to a TSA official in Chicago, Elio Montenegro.
"Our inspector was following routine procedure for securing the aircraft that were on the tarmac," Montenegro told ABCNews.com.
The TSA agent was attempting to determine if someone could break into a parked aircraft, according to Montenegro.
Pilots were furious at the TSA misstep.
"The brilliant employees used an instrument located just below the cockpit window that is critical to the operation of the onboard computers," one pilot wrote on an American Eagle internet forum. "They decided this instrument, the TAT probe, would be adequate to use as a ladder," the pilot wrote.
Another pilot wrote the TSA agents, "are now doing things to our aircraft that may put our lives, and the lives of our passengers at risk."
The TSA has been conducting such overnight spot checks at airports around the country.
Another airline, Mesa Air Group, told its employees earlier this month that "48 percent of all TSA investigations involving Mesa Air Group involve a failure to maintain area/aircraft security."
Mesa said it was imposing a "zero-tolerance" policy for such violations, threatening employees with dismissal.
Eric Longabardi is a freelance journalist who is a frequent contributor to the ABCNews.com investigative page.