President-elect Donald Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE has selected retired Gen. John Kelly as his pick to head the Department of Homeland Security, a high level transition source confirmed to The Hill on Wednesday.

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Kelly will become the third retired general to be tapped for the incoming administration. Retired Gen. James Mattis was Trump’s pick for Defense secretary and retired Lt. Gen. Michael Flynn was his national security adviser pick.

CBS News and the New York Times first reported the pick.

The Washington Post reported that Kelly was selected because of his expertise on the U.S. southern border from his time leading Southern Command. Like Trump, he has warned of the threat of terrorists and drug smugglers crossing into the country from Mexico.

Kelly headed the U.S. Southern Command until retiring in January after more than 45 years in the Marine Corps.

He served in combat in Iraq and spent a year as the head of U.S. forces in that country. Kelly's son, Lt. Robert Michael Kelly, was killed in 2010 when he stepped on a mine in Afghanistan while leading a platoon of Marines.

As the head of Southern Command, Kelly oversaw the prison at Guantanamo Bay, Cuba and publicly butted heads with the Obama administration over its plan to close it.

Jonathan Swan contributed.

- This report was updated at 12:40 p.m.