FILE - In this May 5, 1995, file photo, a worker at the Alfred P. Murrah Federal Building in Oklahoma City, places flowers and memorial items at the scene of the April 19 deadly car-bombing. More than 15 years after a fertilizer bomb was used to blow up the building, killing 168 people, the federal government is proposing to regulate the sale and transfer of the chemical ammonium nitrate. The proposal comes nearly four years after Congress gave the Homeland Security Department the authority to develop a program to regulate the compound. (AP Photo/J. Pat Carter)