The Department of Homeland Security was established in 2002 in the wake of the 9/11 attack on the US in 2001.
The federal department consists of several agencies, including Customs and Border Protection (CBP), Immigration and Customs Enforcement (ICE), the Transportation Security Administration (TSA), Coast Guard (USCG), Secret Service (USSS) and Federal Emergency Management Agency (FEMA).
DHS has faced increased criticism in recent months, particularly in the wake of two US citizens – Renee Good and Alex Pretti – being killed by federal immigration officials during protests in Minnesota.
Noem and CPB chief Greg Bovino, who announced his retirement on Monday, had described Good and Pretti as domestic terrorists immediately after they were shot.
Trump replaced Noem, who was overseeing Operation Metro Surge in the North Star State, with border tsar Tom Homan, who quickly wound down operations there.
Weeks later, Noem sat for tense congressional hearings after the operation.
Shortly after that, Trump announced he was sacking Noem and reassigning her to the newly established position of Special Envoy for The Shield of the Americas – a “new Security Initiative in the Western Hemisphere”.