The major U.S. airlines seem to be leading the way as prices increase in airfare throughout the country. The industry-wide hike comes as the U.S. Justice Department is investigating the possibility of the carriers conspiring to keep the costs of air travel high.

According to a report from CNBC, three of the country's biggest airlines have already increased the fares. Delta Air Lines hiked their prices by up to $4 one-way and Southwest Airlines followed after with $3 one-way increase, FareCompare revealed on Tuesday.

American Airlines matched their rivals with $3 hike one-way for all domestic flights, the report from CNBC said. JetBlue and Virgin America are also confirmed to be part of the industry-wide increase. The last large-scale price hike in the industry was reportedly last June.

It's not uncommon to see airlines adjusting prices throughout the year, but it is reportedly unusual to see increases for all domestic travel and an industry-wide match. The U.S. Justice Department is currently undergoing an investigation on the matter, seeing if the airlines worked together with "plans to limit flights".

JPMorgan analyst Jamie Baker spoke out on the price hikes in a research note recently.

"We are confident in full industry-matching over the next several hours," Baker said. "Just as Southwest often plays the spoiler role, competitors consistently follow Southwest's lead."

Meanwhile, Southwest released an official statement addressing the "modest" hike, according to a report from ABC News.

"Southwest Airlines implemented a modest increase of $3 each way Monday night in markets across our system. Southwest Airlines is the nation's low fare leader and has been for nearly 45 years," the statement said. "In addition to our everyday low fares, we don't nickel-and-dime our customers with hidden fees: we never charge change fees, and Southwest is the only domestic airline that allows every customer to check two bags free of charge (weight and size limitations apply)."

Although the price hike is grabbing headlines as it is likely permanent, the post-holiday season is bringing with it a surge of good deals and cheap flight opportunities. A report from TIME listed the different sale options from airlines across the board.

JetBlue's Big Winter Sale is a good chance for travelers to grab seats this lean travel season as they can book for cheap fares until Jan. 6. Travel period extends from January to April, on limited days. Frontier Airlines, Spirit Airlines and Southwest Airlines also offer a slew of great flights on sale for the first stretch of 2016.