Sept 18 (Reuters) - Alaska Airlines said Wednesday it has completed its $1.9 billion acquisition of Hawaiian Airlines after it reached agreement with the U.S. Transportation Department.

The airlines on Tuesday agreed to maintain key Hawaiian routes and adopt consumer protections under an agreement that will last six years. The Justice Department in August opted not to block the deal that was announced in December by Alaska, the fifth-largest domestic U.S. airline, to merge with Hawaiian, the 10th-largest carrier.

(Reporting by David Shepardson; Editing by Chizu Nomiyama)

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