The defense attorney for 22-year-old Skyler Shepherd argued Thursday that 49-year-old Esther Fujimoto put herself in danger by swimming in open water on Aug. 21, 2011.
Utah medical examiner forensic scientist Suzanne Miles testified that she couldn't detect any blood from the boat's propeller or internal engine parts.
Prosecutors say the men erased any evidence by quickly wiping down the sleek power boat.
Shepherd has said he wasn't driving when the boat struck the University of Utah scientist.
Two other men face a separate trial later, all charged with failing to render aid, reckless endangerment and obstructing justice.
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