In November 1978, Deputy Rebecca Thornfield of Milbrook County, Montana, vanished without a trace. Her patrol car was found abandoned on a dirt road, the engine still running, radio buzzing with unanswered calls. But of Rebecca herself, there was nothing—no weapon, no badge, no duty belt. For years, the official explanation was that she had simply left, run away, or gotten lost. Her family never believed it.
Sixteen years later, in 1994, that cold case suddenly came roaring back to life.
Inside the walls of Deer Lodge State Prison, FBI agent James Valdez sat across from a prisoner named Darren Mulahe. Convicted of robbery and assault, Mulahe was no stranger to trouble. But this time, he wasn’t trying to bargain for freedom—he was dying of pancreatic cancer. And he had a secret he said he could no longer carry.
“I’ve been keeping this for 16 years,” Mulahe confessed. “But I can’t take it to hell with me. I saw who killed her. And I know where her belt is.”
For Agent Valdez, the words were almost too much to process. Deputy Thornfield’s disappearance had long been one of Montana’s most haunting mysteries. Now, a dying convict was claiming he had proof—not only of her death but of a conspiracy to cover it up.
According to Mulahe, the night Thornfield vanished, he had been running moonshine through the woods near Cedar Creek when he heard three gunshots. Curious, he crept closer. What he saw would stay with him for the rest of his life: Sheriff Cornelius Brennan, the very man leading the investigation, standing over Thornfield’s lifeless body.
“She was face down in the dirt, blood around her head,” Mulahe said. “Brennan looked calm. Too calm. Like it was nothing.”
But the story grew darker. A black government sedan arrived, and two men stepped out. Together, they and Sheriff Brennan discussed what to do, eventually loading Thornfield’s body into the car. Brennan, Mulahe claimed, stripped her of her duty gear, dumped it in the woods, and staged her patrol car to look like an abandoned vehicle.
Mulahe admitted he returned later and found Thornfield’s belt, snagged on a tree branch. He took it, not knowing why—only that one day, he might need it as proof. For 16 years, he kept it hidden inside a rented storage unit in Billings.
The implications were explosive. If true, the man trusted to uphold the law had murdered his own deputy and buried the truth with help from others in power.
For Thornfield’s family, the news was both devastating and strangely hopeful. Her older sister, Margaret “Maggie” Walsh, had never accepted the idea that Rebecca had run away. When the FBI contacted her in 1994, she finally heard words she had waited nearly two decades to hear: “We have new evidence.”
Maggie remembered Brennan well—his cold eyes, his dismissive tone when she begged him to keep searching. “He told us Rebecca had probably run off with a man. That wasn’t her. She was dedicated, responsible. She was about to get married. She loved her job.”
Now, with a recovered belt and a sworn statement, the case was reopened.
Working alongside the Montana Department of Justice, Valdez and Detective Sarah Chen began piecing together a puzzle that had been deliberately scattered. They tracked down former deputies from 1978, including Ray Garrison, now the police chief in Cedar Falls. For the first time, men who had once served under Sheriff Brennan were being pressed to revisit the night one of their own vanished.
Inside Milbrook County, a new sheriff, Robert Crow Feather, pledged his cooperation. Unlike Brennan, he spoke plainly: “Justice doesn’t have an expiration date. If crimes were committed, we need to uncover them.”
The recovered belt was sent to the FBI lab for testing, and investigators prepared to dig deeper into Brennan’s past. For Maggie, each step forward was a reminder of both loss and possibility. “Somewhere out there, my sister’s body is waiting to be found,” she said. “And somewhere, the people who did this think they got away with murder. But they didn’t.”
As the small town of Cedar Falls faced the prospect of long-buried secrets coming to light, one thing became clear: the story of Deputy Rebecca Thornfield wasn’t over. It had just begun again.
Sixteen years of silence had been broken by the unlikeliest of voices—a dying convict who chose to unburden himself before it was too late. Whether his confession would lead to justice was still uncertain. But one truth had already emerged: Milbrook County’s darkest night was no longer hidden.
