Summary: Events preceding and following the attacks of September 11, 2001, spotlighted ineffective information sharing, particularly related to intelligence and law enforcement activities, as a serious weakness. Poor information sharing hinders effectively identifying vulnerabilities and coordinating efforts to detect attacks. GAO monitored the Internal Revenue Service's (IRS) efforts to enhance the security of the tax filing process, to study how terrorist-related threat information is shared with IRS. The Federal Bureau of Investigation (FBI) uses task forces and electronic means to share terrorist-related threat information with the Treasury Inspector General for Tax Administration (TIGTA). More specifically, it shares information through its involvement with TIGTA and others in Joint Terrorism Task Forces and through electronic arrangements such as the National Threat Warning System. For its part, TIGTA uses formal communications to disseminate threat information to IRS. TIGTA and IRS officials were satisfied with the FBI's and TIGTA's information-sharing procedures, respectively.