A COUPLE'S MOUNTAIN HIKE TURNS INTO A TRAGEDY - WAS A GIRLFRIEND'S DEADLY FALL AN ACCIDENT OR SOMETHING SINISTER?

"48 HOURS" INVESTIGATES IN "TRAIL OF TEARS"

Saturday, March 5, 2016, 10:00 PM, ET/PT

Before Rhonda Casto and her boyfriend Stephen Nichols headed to the Columbia River Gorge in Oregon for a hike, she joked to a friend that Nichols was either "going to propose to me or kill me." Little did the friend know that Casto wouldn't return from the hike alive.

Peter Van Sant and 48 HOURS investigate Casto's death, the police hunt for clues to what happened, and the case against Nichols in "Trail of Tears" to be broadcast March 5 (10:00 PM, ET/PT) on the CBS Television Network. The police investigation into what happened raised questions about the couple's relationship, allegations of a love triangle and strained family dynamics.

On March 16, 2009, Casto was a 23-year-old mother of a 9-month-old girl. It was also the last day of the fledgling model's life. Somewhere along the trail she slipped or fell, falling 150 feet before hitting solid ground.

"It was a tragic accident," Nichols says.

Nichols said he tried to resuscitate her. He called 911. He told the operator Casto had been acting high, being silly and running on the slippery trail before she fell. Police began investigating Casto's death. An autopsy seemed to support Nichols' version of events. Authorities declined to declare the death an accident, but Nichols wasn't charged with any crime.

Meanwhile, Nichols had started a new life, was raising their daughter and teaching English in China. All along, Rhonda's mother says she kept asking police questions. The investigation went quiet until 2014, when a secret grand jury heard evidence and indicted Stephen for murder. He was arrested in San Francisco on a trip home. He pleaded not guilty.

"I love Rhonda," Nichols tells Van Sant. "I will always love Rhonda. I fully expected to spend the rest of my life with her. I have never done anything to harm her."

Nichols' attorney, Mike Arnold, who recently made headlines representing Ammon Bundy, the leader of the armed occupation of Oregon's Malheur National Wildlife Refuge, maintains Casto's death was an accident.

"We know people fall every day in their homes. Imagine adding exhaustion and weather to that situation," Arnold says. "Anytime you get away from a trailhead, you got to keep in mind you're at the mercy of the outdoors."

When Casto died, she was the fourth fatality in the Gorge trails in sixth months.

"To me, the evidence, the physical evidence and the science shows us it's an accident," Arnold says.

Casto's mother, Julia Simmons, thinks otherwise. "I'm positive that he killed her," Simmons says.

What happened in the Columbia River Gorge that day? And could investigators put together a convincing murder case against Nichols? Van Sant and 48 HOURS report the story through interviews with Casto's friends and family, Nichols and his attorney. 48 HOURS: "Trail of Tears" is produced by Susan Mallie and Leslie Neigher. Richard Barber is the producer-editor. Michelle Fanucci is the field producer. Mike Baluzy and Michael Mongulla are the editors. Anthony Batson is the senior broadcast producer. Susan Zirinsky is the senior executive producer.

48 HOURS: "Trail of Tears" is the second part of a Saturday double feature.

The first, 48 HOURS: LIVE TO TELL: "Sole Survivor," to be broadcast at 9:00 PM, PT/ET, is the remarkable story of a young woman whose family was murdered by a random intruder. 48 HOURS: LIVE TO TELL: "Sole Survivor" is produced by Producer Ruth Chenetz. Doreen Schechter, Joan Adelman and Grayce Arlotta-Berner are the editors. Judy Tygard is the series creator and senior producer. Susan Zirinsky is the senior executive producer.

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