Immigration and Customs Enforcement's (ICE) mission is to prevent terrorist attacks within the United States and reduce the vulnerability of the United States to terrorism while ensuring its mandated customs, immigration, and federal protective enforcement functions are not diminished. The ICE Office of Investigations (OI) supports that mission by investigating customs and immigration violations. This testimony addresses the following key questions that were answered in GAO-06-48SU, a restricted report issued with the same title: (1) What structure and activities has OI adopted to address its mission? (2) In fiscal year 2004 and the first half of fiscal year 2005, how did OI use its investigative resources to achieve its goals? (3) How does OI ensure that its resource use contributes to its ability to prevent the exploitation of systemic vulnerabilities in customs and immigration systems?