Because of the increased deployment of resources and personnel, the number of DEA regulatory investigations of the distribution of controlled prescription drugs more than tripled between fiscal years 2009 and 2010. DEA also conducted outreach to specific registrant types in order to inform them of their regulatory responsibilities and prepare them for regulatory investigations. DEA has taken steps to ensure that investigators follow policies and procedures for such investigations, but it could better assess how its efforts are reducing the diversion of prescription drugs. DEA has established internal controls for guiding, training, and oversight of investigations, and DEA monitors the quality of investigations through a combination of direct supervisory reviews, self-inspections, and on-site internal inspections by DEA's Office of Inspections. GAO recommends that DEA reassess the program's performance measures to better link them to the goal of reducing diversion. GAO did not agree with this recommendation; however, GAO continues to believe the outcome measures could be improved as discussed in the report. GAO reviewed DEA policies and procedures; and it interviewed DEA, State, and local officials at 11 locations that were selected on the basis of volume of cases handled, geographic diversity, and other considerations. 2 tables and 3 figures