In three decades as an undercover operative for the Bureau of Alcohol, Tobacco, Firearms and Explosives, Vincent A. Cefalu risked his life investigating groups from outlaw motorcycle gangs to the Ku Klux Klan. Cefalu spoke with us about the undercover moments that frightened him most.

To better understand package bombs and the serial bombers who employ them, like Austin serial bomber Mark Conditt, A&E True Crime spoke with Tina Sherrow, a retired bomb technician and certified explosives specialist for the ATF.