Implementation of Public Law 104-132, the Antiterrorism and Effective Death Penalty Act of 1996, Relating to the Marking of Plastic Explosives for the Purpose of Detection (96R-029P), Final Rule, Treasury Decision [open pdf - 26KB]
From the Summary: "This final rule implements certain provisions of the Antiterrorism and Effective Death Penalty Act of 1996 (Pub. L. 104-132). These regulations implement the law by requiring detection agents for plastic explosives. The final rule also authorizes the use of four specific detection agents to mark plastic explosives and provides for the designation of other detection agents."
Federal Register: http://www.federalregister.gov/