David Cahill

David Cahill testified for the prosecution on Day 38 as a Massachusetts State Police lieutenant and firearms examiner. He described collecting shooting evidence at MIT and in Watertown and identified five MIT-related projectiles and 56 Watertown 9 mm casings as fired by the Ruger P95 recovered on Laurel Street. His findings connected evidence from the two shootings to the same weapon, not to a particular person firing it.

Before his testimony, the defense requested voir dire on the reliability of his anticipated opinions and sought postponement to prepare. The court allowed voir dire but decided to proceed, stating that admissibility depended on the foundation Cahill established. That exchange did not include a final ruling admitting his opinions.

On direct examination by Steven D. Mellin, Cahill explained his training, laboratory review procedures, and microscopic comparisons. He distinguished the Ruger casings from casings attributed to submitted police firearms. He acknowledged that deformation or insufficient markings can prevent identification, while maintaining that the MIT projectiles were identifiable.

On cross-examination by Timothy G. Watkins, Cahill said he had not received fingerprint reports and did not know whether comparisons had been made to either Tsarnaev brother. He clarified that 16 rifle casings included in the police total came from a separate incident four blocks away. He agreed that officers fired at the passing Mercedes but could not match its 32 recovered projectiles to police weapons. Casing locations indicated general officer positions, not firing directions. The examination did not establish the source of Richard Donohue's injury, and the court barred a proposed inference about where an officer's missed shot would land.

On redirect, Cahill explained that casings can be lost, carried away in vehicle tires, or collected as souvenirs. These were possible explanations, not findings that particular casings had been removed. He separately confirmed that a projectile recovered inside a Watertown police cruiser had passed through its windshield and matched the gun recovered at the scene.