The reports, filed Thursday, indicate Bulger may have bought the firearms at gun shows, where sellers aren't required to complete background checks or ask for identification, in Nevada and Utah.
The Bureau of Alcohol, Tobacco, Firearms and Explosives has been tracing 29 weapons the FBI found in Bulger's apartment after he and his girlfriend, Catherine Greig, were arrested in June 2011 in Santa Monica, Calif., The Boston Globe reported.
Bulger, 83, is charged with 19 murders in the 1970s and 1980s. His trial is to begin in June. A judge is to rule next week whether Bulger should have immunity for his crimes as a longtime FBI informant, the Globe said.
New Bedford, Mass., Mayor Jon Mitchell, part of a group of mayors pushing for stricter gun control, said a background check may have not stopped Bulger but that more gun restrictions are needed, especially at gun shows and for private sales.