Three people were killed and more than 260 injured in the April 15 bombing.
The Los Angeles Times reports the bulletin was sent to state and local law enforcement officials late Tuesday. The Times said investigators have completed preliminary reconstruction of the bombs, which were more sophisticated that initially believed.
The pressure cooker bombs were made from an explosive mixture that included nitrate and perchlorate oxidizers and were packed with shrapnel, the bulletin said.
"Based on preliminary analysis of recovered evidence, each device likely incorporated an electrical fusing system using components from remote control toy cars such as a transmitter and receiver pair operating at 2.4 GHz, an electronic speed control used as the switch mechanism and sub-C rechargeable battery packs at the power source," an official told the Times the bulletin said.