Former investigator: bomb inside RV that exploded on Christmas Day was likely pyrotechnics A retired ATF investigator said the downtown bomb could've been built with materials anyone can buy. and last updated 2021-01-07 21:57:42-05 NASHVILLE, Tenn. (WTVF) â A retired ATF investigator said the downtown bomb could've been built with materials anyone can buy. The investigator, who retired from the ATF less than a year ago, said his theory is the bomb was, at least in part, made out of pyrotechnics. David Hyche spent 31 years at the ATF. He is an explosives expert and spent much of his career doing post-explosion investigations.