Retired four-star Marine Gen. John Kelly, the former head of U.S. Southern Command, who has met with president-elect Trump several times in recent weeks, has emerged as a strong contender for secretary of homeland security, ABC News has learned.

Rep. Michael McCaul, chairman of the House Homeland Security Committee, has been another contender.

Kelly's name was confirmed by multiple senior Trump transition sources as garnering serious consideration.

The DHS position has been one of the trickiest to fill: the agency is one of the largest in the federal government and is one of the most challenging posts, given the broad and evolving security threats.