Most of the DHS employees are part of pre-existing agencies that were combined to form DHS.
"187 federal agencies and departments, including the United States National Guard, the Federal Emergency Management Agency, the United States Coast Guard, Customs and Border Protection, U.S. Immigration and Customs Enforcement, United States Citizenship and Immigration Services, the United States Secret Service, the Transportation Security Administration, the 14 agencies that constitute the U.S. intelligence community and Civil Air Patrol"
Edit: removed FBI reference, posted subsumed agencies as per Wikipedia...