Alaska Air Group has agreed to extend the review period for the proposed $1.9 billion acquisition of rival Hawaiian Airlines amid antitrust scrutiny from the U.S. Department of Justice, the carrier said in a filing today.

“Our airlines have mutually agreed with the DOJ on a brief extension of the formal review period until August 15,” Alaska Air Group said.

The deal, announced last year, has been under antitrust review and both carriers said in March they “have been working cooperatively with the DOJ and expect to continue to do so”.

Shortages of new planes, jet engines and pilots have spurred U.S. airlines to pursue growth through acquisitions, putting them in the crosshairs of anti-trust regulators who have generally been wary of mergers between smaller airlines.

The takeover would give Alaska Air control of more than 50% of the market for flights to Hawaii, one of the world’s top holiday designations.

The airlines, in March, entered a timing agreement with the DOJ, under which the carriers agreed not to consummate the deal within 90 days after submitting antitrust-related information.