The following is from Oct. 24, 2013 Congressional Research Service report: FY2014 Appropriations Lapse and the Department of Homeland Security: Impact and Legislation.
Absent legislation providing appropriations for the Department of Homeland Security (DHS) for FY2014, the Department implemented a shutdown furlough on October 1, 2013. Operations of different components were affected to varying degrees by the shutdown. While an estimated 31,295 employees were furloughed, roughly 85% of the department’s workforce continued with their duties that day, due to exceptions identified in current interpretations of law. Some DHS employees were recalled to work after the furloughs began on the basis of unanticipated needs (such as disaster preparedness activities) or the enactment of appropriations legislation that temporarily covered personnel costs.