Timothy E. Dowd

Timothy E. Dowd testified for the prosecution on Day 38 about mapping evidence at the Laurel Street confrontation scene in Watertown. A Massachusetts State Police sergeant assigned to collision analysis and reconstruction, he described GPS readings that he said were accurate within 3 centimeters, plotted against aerial images using AutoCAD. He recorded items identified by Trooper Colleen Tanguay and believed most had already been collected, leaving placards. Two diagrams, a field-sketch page, and a scene photograph were admitted without defense objection.

Dowd distinguished measured evidence locations from vehicles positioned using Trooper Michael George's sketch. On direct examination, he acknowledged that the sketch represented driveway-car positions more accurately than his diagram. Using a photograph and his memory, he identified the explosion site and confirmed a direct line and sufficient space between vehicles for a pressure cooker to reach the Honda. That testimony described scene geometry, not who threw the pressure cooker or its actual path.

Under cross-examination by Timothy G. Watkins, Dowd acknowledged inaccurate vehicle placement where tire-location GPS readings had not been taken. He said he had given prosecutors an original diagram containing the recorded locations before it was broken into narrower maps. He believed the total was approximately 448 points, not 448 evidence items. He also explained that he recorded placard numbers without necessarily knowing what they represented. Shadows prevented him from clearly locating a diagrammed driveway in an aerial photograph, and the comparison did not establish a definite mismatch.

Watkins sought to explore whether the scene locations could rule out Tamerlan Tsarnaev's gun as the source of Officer Donohue's injury. Prosecutors said they could not identify who shot Donohue and would not argue that either brother did. The court treated shooter identification as speculative, sustained relevance objections to the injury inquiry and a proposed Google Earth comparison, but allowed Dowd to mark two locations on an aerial photograph. The court later declined a defense request to recall him on the injury issue.