The Justice Department is investigating whether multiple airlines joined forces and colluded by limiting available seats, allowing them to raise prices, the Associated Press reported Wednesday.

A spokeswoman from the Justice Department confirmed to the news service that the agency is investigating "unlawful coordination" among major airlines, but did not say which airlines are being probed.

The investigation is focused on whether the carriers colluded illegally and informed each other as to "how quickly they would add new flights, routes and extra seats," the AP reported.

Such collusion could be responsible for rising airfares, which increased by 13 percent from 2009-14, according to the Bureau of Transportation Statistics.

The AP reported that major U.S. airlines received a letter from the Justice Department Tuesday that requested copies of any communications about airplane-capacity strategy among airline companies, Wall Street executives and company shareholders.

A number of mergers among domestic airlines have drastically changed the airline industry since 2008, allowing a small number of airlines to control much of the industry. More than 80 percent of airline seats for domestic flights are owned by American Airlines, Delta Air Lines, Southwest Airlines and United, according to AP.

Since 2008, the industry also has seen immense profits. U.S. airline companies raked in $19.7 billion total over the past two years.