David McCollam

David McCollam testified for the prosecution on Day 40 as an FBI forensic chemist assigned to the Explosives Unit. He described laboratory testing that found Boylston Street blast residues consistent with burned pyrotechnic material and Watertown bulk powder consistent with pyrotechnic flash-powder-type material. He also reported pyrotechnic residues in Norfolk Street filters and on gloves from a Honda CR-V, and black powder grains in four Norfolk Street samples.

McCollam explained that fireworks were one common source of pyrotechnic material, not the only source, and discussed approximate extraction yields and powder transfer. He said his unit had not quantified the residues, he had not weighed the bulk powder, and the residue evidence did not identify a specific assembly location. After defense objections, the court excluded an undisclosed expert opinion comparing the trace-residue amounts with the explosives involved.

On cross-examination by Timothy G. Watkins, McCollam distinguished the pyrotechnic findings on gloves Q667 and Q668 from the more limited chemical findings on Q671. He explained that a hobby fuse contained bulk low-explosive material and that a can lid carried chemicals commonly found in some double-based smokeless powders. He acknowledged having no firsthand collection knowledge and reported that vacuum filters identified in laboratory records as coming from a UMass Dartmouth dormitory room screened negative for the typical explosive-residue traces tested. That result did not establish a broader conclusion about activities in the room.

On redirect, McCollam contrasted trace residues with intact explosive materials. His confirmation of intact fireworks from Pine Dale Hall remained conditional on the origin supplied to the laboratory. The court sustained an objection to a question about production location.