From the Document: "On December 29, 1975, a bomb-blast ripped through New York's LaGuardia airport, killing 11 persons and injuring some 15 others. [...] The increase in bombing incidents in this country and the increase in explosives thefts is of great concern to all segments of the American public. ATF [Bureau of Alcohol, Tobacco, and Firearms] believes that you, as members of the explosives industry and as dealers in and users of its products, are particularly concerned in this regard; we urge your cooperation, therefore, in reporting thefts of explosives promptly and in providing greater security for your premises and magazines."
Report No. 76-3
Bureau of Alcohol, Tobacco, Firearms and Explosives: https://www.atf.gov/