The lowest number of airport security screeners in five years and the highest number of travelers in the same time period are boosting security wait times to nerve-wracking levels at O’Hare Airport and elsewhere, officials said Monday.

Others blamed more subtle factors for the serpentine O’Hare lines that forced American Airlines to rebook 450 passengers and pushed wait times to as long as 2½ hours on Sunday night.

More passengers are bringing luggage through security to avoid checked-bag fees and more airlines are booking to capacity, which also could contribute to the long, winding lines, said Jonathan Grella of the U.S. Travel Association.

All that was little comfort to O’Hare travelers on Sunday night and Monday morning.

Julio Pacon, 46, of Montreal, estimated that he joined a frustratingly long O’Hare security line of about 300 passengers about 7:30 p.m. Sunday. He had arrived from South Korea for an 8:50 p.m. United Airlines connecting flight to Montreal.

“The lines were so humongous … that we spent the night at the Hilton,’’ Pacon said Monday morning. United had warned him he would not make his connection and offered him 50 percent off a hotel room for the night, which ended up costing him, his wife and two children $150, he said.

Pacon has since alerted all his coworkers at a Montreal-based food export-import company to bypass O’Hare, especially for connecting flights.

“I told everyone to avoid Chicago because you’re not going to make any meeting in the United States,” Pacon said.

But the long lines have dogged O’Hare – and other airports — far before this weekend. The situation has become so common that the Airlines for America trade group earlier this month asked frustrated travelers to post their beefs on Twitter under #IHateTheWait.

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Last Saturday, a Los Angeles Airport traveler tweeted: “If you are flying United out of LAX, get there at LEAST two hours early. The security line this morning was insane.”

About 4,500 American passengers have missed their departing O’Hare flights since February because of backlogs at security lines, said Leslie Scott, an American Airlines spokeswoman. On Sunday night, American offered cots to less than 100 travelers.

As the busy Memorial Day weekend approaches, help is on the way.

American has hired workers to remind travelers at security checkpoints to take off their shoes, unpack their laptops, and remove liquids from their luggage, Scott said. The new workers should be in place well before the traditional beginning of the summer travel season hits on May 26.

“There’s a financial impact to us if people miss their flights, so we are trying to do everything we can,’’ Scott said.

United Airlines hired more help with security and non-security functions earlier this month and is in the midst of a second wave of such hires, United spokesman Charles Hobart said.

“It’s an investment in our customers because these wait times are unacceptable,’’ Hobart said.

Meanwhile, battling mounting complaints nationwide, the Transportation Safety Administration has stepped up the hiring of 768 TSA officers from September to this month, TSA spokesman Michael McCarthy said.

TSA also will be increasing its overtime and part-time hours and beefing up its use of bomb-sniffing dogs, McCarthy said.

McCarthy tied long security lines that have triggered the ire of some congressmen to the smallest number of TSA front-line officers in five years — and the highest number of departing passengers requiring screening in the same time period.

TSA screeners nationwide numbered 45,000 in fiscal year 2011. They are now at 42,525. At O’Hare, TSA full-time and part-time workers dropped from 2,045 in 2012 to 1,932 last year, McCarthy said.

Meanwhile, the TSA predicts 740 million passengers nationwide will wind through departure screening this fiscal year, a five-year record, McCarthy said.

“TSA would screen about 1.8 million passengers in a given day last year nationwide. This year we are seeing about 2 million in a given day, so our officers are screening what we would have typically seen on a busy holiday weekend,” McCarthy said.

Contributing to the problem is that TSA predicted that close to 50 percent of its travelers would go through its PreCheck expedited screening by now, but the current number is only about 30 percent, McCarthy said. PreCheck requires travelers to submit in advance to a background check and fingerprints and to pay an $85 fee.

Following a scathing Inspector General audit earlier this year, the TSA removed some people from its PreCheck lanes, resulting in fewer PreCheck travelers, McCarthy said.

In addition, TSA has had a “renewed emphasis on rigorous screening” since the IG report, McCarthy said.

The March Brussels airport terrorist bombings proved the need to stay vigilant.

“TSA’s primary focus is the current threat environment, as the American transportation system remains a high-value target for terrorists,” McCarthy said.

Security lines also may be delayed by increased passengers going through them with luggage to avoid bag-check fees, Grella said.

To attack that problem, a U.S. Senate bill would block airlines from imposing fees, including on checked bags, that “are not reasonable and proportional to the costs incurred.”

In announcing the measure amid the airline industry’s most profitable year in decades, Sen. Edward Markey, D-Mass., said it would stop “fee gouging.” However, American’s Scott pointed to long TSA lines at Midway’s Southwest Airlines, which does not charge for bags.

Airlines that book at or near capacity also could be adding to screening lines, Grella said.

Grella advocates greater use of TSA’s PreCheck program so that TSA can focus on “those they know less about rather than those they know more about.’’

Ideally, Grella said, PreCheck will gradually become as commonplace as electronic tollway payment lanes, which evolved from one lane to the majority of lanes across many tollways nationwide.

TSA was advising travelers Monday to arrive two hours early for domestic flights, and three hours in advance of international flights.

Alarmed by news of up to three-hour waits at O’Hare on Sunday night and up to two hours Monday morning, Maureen Curtis-Cooper snapped into action Monday afternoon.

“We ended our business meeting early,’’ said Curtis-Cooper, 64, as she headed home to Massachusetts through O’Hare. “We heard on the news it was a three-hour wait.”

By Monday afternoon, O’Hare Airport staff said, security lines were back to normal, taking about 35 minutes.