A jury heard testimony from the FBI and an expert on tires on Monday as the James Wells murder trial continues into its third week.
Video surveillance and a flat tire alibi were both examined in front of the jury.
Federal prosecutors say Wells killed two of his co-workers in April of 2012 and returned home to craft an alibi involving a flat tire.
The defense says Wells returned home to repair his tire and called a friend for help, but the prosecution is trying to show Wells drove to work at a Cost Guard Communications Station on Kodiak Island and shot Electrician's Mate First Class James Hopkins and retired Chief Boatswain's Mate Richard Belisle.
An expert witness specializing in tire forensics testified that the nail in Wells' tire hadn't been driven on and had been inserted manually, possibly with a nail gun.
The defense questioned the accuracy of lab tests conducted in temperatures over 100 degrees and how easily a bent nail could be fired from a nail gun into a tire.
Jurors also watched video surveillance that prosecutors say shows a timeline of when Wells left home and drove to Coast Guard Communications Station Kodiak.
The defense says the cameras had blind spots and anyone could have approached the station unseen.