"It has been a great honor to serve our country as FEMA Administrator for the past two years", said Long in the release.
"I leave knowing the agency is in good hands", Long said. But the use of government vehicles for home-to-work travel was not officially authorized, and that practice was eliminated in April.
In a statement, Long said FEMA had provided assistance on "more than 200 declared disasters" and thanked President Donald Trump, Vice President Mike Pence and Homeland Security Secretary Kirstjen Nielsen, whom he said "have been extremely supportive of me, the FEMA workforce and our mission".More news: Former 'American Idol' Contestant Facing Federal Drug Charges
Long had been investigated by the Department of Homeland Security's internal watchdog a year ago over allegations that he inappropriately used government vehicles to travel to his home in North Carolina.
In the post since June 2017, he has led the response to several extreme natural disasters, including the heavily criticised operation in Puerto Rico after a devastating hurricane.
But Long's time as the head of FEMA was also controversial.More news: Pentagon fears China, Russia pose challenge to US space capabilities
"The president and his entire administration provided unprecedented support to the agency as we led the nation through the historic 2017 hurricane and wildfire season", he said.
Long also served as FEMA administrator through Hurricane Harvey, which rocked Houston, Hurricanes Irma and Michael's landfall in Florida, wildfires in California, and more. FEMA's response to Maria was widely criticized as slow and inadequate. "We've got to figure out how to build better baseline capabilities at all levels of government".More news: North Atlantic Treaty Organisation chief claims alliance preparing defensive measures for 'INF-free world'
Long said FEMA's No. 2 official, Pete Gaynor, would take over on a temporary basis when he departs. It was not immediately clear when he would leave the agency.
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