On Sunday, “the lines were in excess of 2½ hours,” and the airline had to provide cots for customers who missed the last flights to their destinations. American Airlines held 30 flights at O’Hare Sunday to allow delayed passengers to scramble on board, Scott said.

Last week, about 795 passengers missed flights at O’Hare because of lengthy waits, and that number totaled more than 4,500 people as of February, when TSA staffing problems emerged.

The TSA has reduced its screeners by about 10 percent, or 4,622 people, in the last three years in hopes passengers would sign up for PreCheck, a program that expedites security steps for preapproved travelers. But fliers aren’t rushing to enroll and the agency is also losing about 100 employees a week through attrition.

At midmorning Monday, security lines were moving at O’Hare’s Terminal Three, but it was a different story at 7 a.m. with overflow crowds, Scott said, adding that delays begin to mount again as the afternoon rush approaches.

“I arrived early just to be sure,” St. Charles resident Beth Bisbee said en route to Bismarck, N.D. “And, I purposely scheduled a later flight.”

The Chicago Department of Aviation Monday advised fliers to arrive three hours in advance of flights.

Complicating matters, the TSA has tightened security checks at the same time more Americans are traveling.

The agency does expect to receive funding from Congress to hire 768 more screeners by mid-June, although union officials have warned that 6,000 are needed.

• Daily Herald news services contributed to this report.