The Justice Department is investigating whether some of America’s biggest airlines have colluded to keep airfares high. (Daniel Acker/BLOOMBERG)

The Justice Department is investigating whether some of America’s biggest airlines have colluded to keep airfares high, striking at an industry that has posted record profits recently while limiting routes and affordable seats, officials familiar with the matter said Wednesday.

Justice Department spokeswoman Emily Pierce confirmed the probe, saying investigators are looking into “possible unlawful coordination by some airlines,” but she would not name the carriers.

Representatives from Delta Air Lines, Southwest Airlines, American Airlines and United Airlines confirmed they were among those being investigated and said they were complying with Justice Department requests.

Lawmakers and consumer advocates have routinely called for investigations into whether ­airlines, to boost prices, limit the number of tickets they sell, with Sen. Richard Blumenthal ­(D-Conn.) recently alleging widespread “anticompetitive, anticonsumer conduct.”

It’s “Economics 101. Reducing supply with rising demand means increased prices,” Blumenthal said in an interview. “Consumers are suffering rising fares and other added charges that seem to be the result of excessive market power concentrated in too few hands and potential misuse of that power.”

A series of bankruptcies and mega-mergers over the past decade has slimmed the number of major U.S. airlines from nine to four, and those carriers — Delta, Southwest, American and United — fly about 80 percent of all domestic passengers.

The limited competition has helped airlines post some of their biggest profits in history, 14 years after the 9/11 attacks nearly drowned the industry in red ink. In the first quarter of the year, American logged $1.2 billion in profit, its most profitable three months ever.

The investigation is a surprising shift for the Justice Department, which approved the unions in the first place, including the 2013 merger of American and US Airways that created the world’s largest air carrier.

“This is a long time coming,” said Diana Moss, president of the American Antitrust Institute. She said that in meetings of top industry executives, including at last month’s International Air Transport ­Association conference, airlines have continuously signaled “to each other that it was in their joint interest to keep capacity tight and to keep prices high.”

Jean Medina, a spokeswoman for industry group Airlines for America, swatted back claims of collusion, arguing there are “so many options among air carriers,” including on low-cost carriers such as Spirit Airlines, one of the industry’s fastest-growing firms.

“It is customers who decide pricing, voting every day with their wallets on what they value and are willing to pay for,” Medina said, adding that about 222 million passengers are projected to fly this summer, a post-recession high.

“We are confident that the Justice Department will find what we know to be true,” Medina said. “Our members compete vigorously every day, and the traveling public has been the beneficiary.”

U.S. airlines received a letter Tuesday demanding copies of all communications between carriers, their shareholders and investment analysts about their plans for limiting seat capacity, according to the Associated Press, which first reported the investigation.

“We welcome the review,” said American spokesman Joshua Freed, adding that “the industry remains highly competitive with more people flying than ever before.”

Historically low prices for jet fuel have helped the carriers save billions of dollars on their biggest expense, with the four major airlines saving about $3.3 billion on fuel in the first quarter alone, financial filings show.

Yet those savings have not found their way back to passengers. The average domestic flight last year cost $391, the highest price since federal statisticians started tracking fares two decades ago. Adjusted for inflation, fares are at a 12-year high. Airlines have made billions of dollars more on bag-checking, reservation and other added fees.

“It’s hard to understand, with jet fuel prices dropping by 40 percent since last year, why ticket prices haven’t followed,” Sen. Charles E. Schumer (D-N.Y.) said in a statement Wednesday. “We know that when airlines merge, there’s less price competition. What we need now is a top-to-bottom review to ensure consumers aren’t being hurt by industry changes.”

It’s unclear what sparked the investigation, but some said it was not helped by last month’s International Air Transport Association gathering, in which air executives routinely stressed the importance of “capacity discipline” to cut down on costly, competitive price wars.

“The Justice Department doesn’t just launch investigations as fishing expeditions,” said Gene Kimmelman, a former Justice Department antitrust official. “There’s a keen awareness that when they request documents, there’s a significant cost to companies, it’s not easy and there are a lot of expenditures. . . . They have to have a strong reason to believe there may be a violation of law.”

Some industry experts doubted the investigation will ever leave the ground. Bijan Vasigh, a professor of economics and finance at Embry-Riddle Aeronautical University, said reduced air competition had led to climbing ticket prices, reduced flights and denser cabins. However, he said, that was not collusion but rather “a natural consequence of the mergers” that eroded competition.

Vaughn Cordle, a pilot for a major airline and chief analyst for Ionosphere Capital who has been subpoenaed to testify on behalf of consumer groups in past suits against airlines, said “competition is still alive and well” on most routes.

“I don’t think you’re going to find anybody who actually runs [airline fare calculations] to suggest that these guys are colluding,” he said. “They’re going to fail as badly, and look as bad, as those consumer advocate groups who hired all those lawyers to try to block the mergers.”

An investor sell-off triggered by news of the investigation led some airlines’ shares to plunge, including American (down 2.8 percent), United Continental (2.5 percent), Delta (1.9 percent) and Southwest (1.4 percent).

Paul Hudson, president of the traveler advocacy group ­FlyersRights, points to airlines’ reduction of new routes and resistance to cheap tickets as a sign of an untoward airline partnership. “That’s a thinly veiled way of saying, ‘We’re not going to compete,’ ” Hudson said, and “ ‘We need to restrict supply in the face of increased demand.’ ”

Airlines have also fought to block third-party booking sites such as TripAdvisor and Orbitz from posting airfares that could help travelers shop for cheaper tickets. In May, the industry group Travel Technology Association released a study led by a former Justice Department official that said the practice cost passengers more than $6 billion a year and discouraged 40 million travelers from flying because prices seemed too high.

Some in the travel industry worried the investigation could hurt not just airlines but also the businesses that depend on American travelers paying handsomely for their seats in the sky.

“American consumers are already jaded enough about flying,” said Roger Dow, president of the U.S. Travel Association, an industry group. “We’ve been wondering for a while how many more gut punches they could absorb before we see a dip in air-travel demand — and therefore a dip in the related econometrics for cities and businesses across the country.”

Sari Horwitz contributed to this report.