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Retired Marine General John Kelly, 66, is the likeliest candidate to be tapped to lead the Department of Homeland Security, three people close to President-elect Donald Trump's transition process revealed to Politico.

The Department of Homeland Security, established after the Sept. 11 terrorist attacks, helms border and immigration control, both of which are issues Trump had made central to his campaign. Kelly had expressed interest in serving in the incoming administration, whether under Trump or Hillary Clinton; he did not endorse a candidate. In the past, Kelly has clashed with President Obama on the decision to open combat roles to women in the military as well as the administration's plans to close Guantanamo Bay.

After four decades in the military, Kelly recently retired as the chief of U.S. Southern Command, which oversaw military operations in Central and South America. Kelly is also one of the most senior military officers to have lost a child in Iraq or Afghanistan; his son, 2nd Lt. Robert M. Kelly, was killed after stepping on a landmine in Afghanistan in 2010.

Notably, if Trump were to select Kelly, he would be the third general to join the incoming administration, after Lt. Gen. Michael Flynn, who will serve as national security adviser, and retired Gen. James Mattis, who has been nominated for defense secretary. Retired Gen. David Petraeus is also reportedly being considered for secretary of state.

You can read more about Kelly, and his experience and positions, at Politico.