– Marathon security lines over the weekend stranded dozens of people overnight at O’Hare International Airport because of missed flights, as a TSA staffing shortage continues to plague travelers nationwide.

Transportation Security Administration (TSA) lines were more than two hours long Sunday, causing 450 people to miss their flights, according to American Airlines spokeswoman Leslie Scott. Stranded passengers who couldn’t get on a later flight slept on cots at the American terminal.

The lines are being blamed on insufficient TSA staffing because of strengthened security measures, a higher-than-anticipated increase in air traffic and incorrect guesses about how many people would sign up for expedited screening.

The TSA says its plans to hire nearly 800 new officers this month and pay for more part-time workers and overtime. The union that represents security officers said that won’t solve the problem, and that TSA needs 6,000 additional full-time officers.

The situation has been accelerating for months — and is expected to get worse heading into the summer travel season.

“Obviously it’s a frustrating situation for our passengers and we know that,” said Scott, who said 4,500 American Airlines passengers at O’Hare have missed flights since February because of the line problem.

The number of complaints filed with the Department of Transportation reflect this growing frustration. Complaints filed on such topics as courtesy and processing time surged in March to the highest levels in the past year, according to the DOT’s monthly Air Travel Consumer Report released Monday.

Reports complaining about TSA screening times grew more than tenfold, to 513 this past March from 48 in March 2015. Concern about rude screeners increased more than threefold, to 1,012 in March from 294 a year ago.

At MSP

At Minneapolis-St. Paul International Airport, complaints about TSA also jumped significantly from March 2015 to March 2016, said spokesman Patrick Hogan. In March 2015, there were eight complaints about checkpoints; in 2016, there were 55.

Chicago’s Midway Airport was having problems, too. When Erin Walton, of Rockford, Ill., left Midway on Thursday, she and her family stood in line for more than two hours and barely made their flight to Dallas.

They arrived early at the airport in Dallas on Monday so they could make their return flight. They encountered long lines there, too. “We just try to endure,” she said.

The wait problem also hurts airline operations because airlines risk half-empty planes if lines are too long. Airlines sometimes pull people to the front of the line if their flight is leaving soon, or face the difficult choice of delaying a flight and causing a cascade of late arrivals around the country.

The TSA admits it had hoped for an increased use of a four-year-old expedited screening program called PreCheck, with a goal of getting 25 million fliers into the program by 2019.

But only about 7.25 million people have enrolled in prescreening programs such as PreCheck or Global Entry, said a TSA spokesman.

Staff writer Pat Pheifer and Bloomberg News contributed to this report.