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Charts of the day: American Airlines, Southwest, Delta, United Continental

Yahoo Finance is closely tracking airline stocks after the Wall Street Journal reported that the US Justice Department would not bring action against the biggest US airline carriers. The probe was launched nearly two years ago and focused on whether airline giants were coordinating flight capacity in an effort to prop up fares.

The major airlines that received subpoenas when the investigation began in 2015 were Delta Airlines (DAL), American Airlines (AAL), United Airlines (UAL) and Southwest Airlines (LUV).

The investigation by regulators started after the Justice Department permitted two big airline mergers in the past few years, including United and Continental as well as American and US Airways.

Shares of United Airlines are also getting a boost after the airline reported revenue passenger miles (RPM) rose 2.6% in December, driven by a 3.7% rise in domestic RPMs.

On the other hand, American Airlines reported revenue passenger miles fell 0.8% in December, dragged down by a 3% decline in domestic RPMs. Despite the drop, the airline lifted its united revenue guidance. It now expects fourth-quarter revenue per available seat mile to be flat to an increase of 2%, compared to its previous forecast of -1% to +1%.

Earlier this week, Southwest raised its fourth quarter unit revenue guidance to a range of -4% to -3%, up from its prior estimate of -4% to -5%.

For more on the airline sector, check out The Final Round, live today at 4 p.m. EST right here on Yahoo Finance.