The Transportation Security Administration apologized to Chicago — and dispatched dozens of reinforcements — after a security screening nightmare last year at O’Hare Airport that saw passengers miss flights and sleep on cots because of hours-long waits to get through security.

On Monday, City Hall and American Airlines took a small but important step to make certain there is no recurring nightmare during this year’s summer travel crunch.

The Chicago Department of Aviation used $1.8 million in federal grant money to buy four new shuttle buses at $451,000 apiece that are now transporting American passengers arriving on domestic flights at Terminal 3 to the International Terminal.

That will allow those passengers — who account for roughly 60 percent of all domestic-to-international connections — to avoid having to be re-screened before boarding connecting international flights.

The new shuttle bus service will also have side benefits for other overseas travelers, as it will reduce the volume of air travelers passing through the Terminal 5 security checkpoint by as many as 1,000 passengers a day. American is providing the staff to process passengers at pick-up and drop-off locations.

Two buses will provide continuous service each day between 10:30 a.m. and 8 p.m. during the spring, summer and early fall. Passengers will be picked up at two locations at Terminal 3 — near Gates G-17 and K-20 — and dropped off at the M-Concourse of Terminal 5.

“United and Delta have chosen not to participate at this time,” Aviation Department spokeswoman Lauren Huffman said in an email.

The shuttle bus is also aimed at alleviating delays tied to capital projects.

The international terminal is adding nine new gates by 2020 — the first expansion in more than 20 years. American is also building five new gates at Terminal 3 expected to open next year.

Until now, passengers arriving at O’Hare on domestic flights have connected to their international flights by taking the people-mover system. That requires them to exit security, then be re-screened at the TSA security checkpoint in Terminal 5.

Last year, Emanuel credited a “heroic” effort by the TSA for turning a May meltdown at O’Hare into a summer travel success story.

Thanks to an influx of screening officers and canine teams, passengers who missed flights and slept on cots after waiting an average of 98 minutes to get through security breezed through TSA checkpoints during peak travel times in an average of 7 minutes.

On Monday, City Hall reported that the current wait times at O’Hare and Midway is “under 10 minutes, even during peak travel times.” That’s “among the lowest” for big-city airports nationwide, officials said.