48 Hours: "Collision Course" probes deadly crash that was no accident

(CBS) NEW YORK -- Investigating the death in November of 2009 of a young Iraq war vet named Brittney Brashers had Denver homicide detective Troy Bisgard in a quandary...was this death an accident, or was it murder?

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He knew the facts: Brittney, who'd been drinking, had plowed her car into a row of parked vehicles. Her one-time boyfriend, Robbie Walters, was with her in the passenger seat. He was wearing a seatbelt. She was not. He escaped with minor injuries. But she was dead.

But beyond the facts, Bisgard had his suspicions. For one thing, Robbie Walters was married. Brittney, her friends said, wanted nothing more to do with him. And she wanted out of the relationship.

Even more significant, the Medical Examiner said the injuries Brittney got in the crash weren't enough to have killed her. He, too, had his doubts, but he had no choice but to put "undetermined" on the autopsy report.

With that, there was little more Bisgard could do. Except stew. Months passed, and the case haunted him. He got to know Brittney's family, especially her father, who pleaded with him not to give up. Other crimes demanded his time, but the detective couldn't stop thinking about the vivacious Iraq vet, and the distraught family she left behind.

Whenever he had time, Bisgard would contact people who knew Robbie Walters, and eventually that perseverance paid off. Because this was a suspect who couldn't stop talking, and in this age of iPhones and mobile devices with recording capability, that can be a very dangerous thing for a murder suspect to do...

In the end, it would be the suspect's own words that would convince Bisgard that this was no accident - this was murder. Now the question was, would those astonishing tapes convince a jury....

Watch 48 Hours: "Collision Course," Saturday, Oct. 13 at 10 P.M. ET/PT