Cyrus never expected her final moments with her first-born child would be in October 2011, as the sixth-grader lay unresponsive in a hospital room in Potsdam, New York.
She rushed to the hospital that evening thinking Garrett had fallen, "not knowing how bad he was hurt or what had happened," she said.
Cyrus, taking a deep breath, then told "20/20," "He had bruises on his face ... marks on his neck" and the medical team was doing CPR.
Crying, Cyrus told ABC News, "It felt like it was somebody else's life. I felt like I was watching it happen to someone else."
"The doctor was talking to me. And I just remember him looking down at me over the tops of his glasses, telling me that there was nothing else that they could do," she said.
"He's 12 years old. Healthy, athletic. They told me he fell," she said. "And I get there, and they're telling me he's in cardiac arrest. And then they're telling me that he's gone. I had no idea what happened."
She said she stayed in the hospital room with Garrett that night. "I don't even know how long I was in there. Until they came and told me ... I had to come out," she said. "I told him I loved him."
The grieving mother later learned that Garrett was strangled to death. Almost immediately, police focused on her ex-boyfriend Hillary as the prime suspect, but it would take them several years before they arrested him for the murder. After a two-week trial, Hilary was found not guilty this morning by a judge.
Cyrus and Hillary broke up shortly before Garrett's death, and she believes that Hillary blamed Garrett for the breakup.
While Cyrus said she never saw Hillary hit a child, lose his temper with a child or threaten a child during their nearly one year together, she told "20/20" that she has no doubt that Hillary killed her son.
Cyrus said sitting through Hillary's trial this month has been "very difficult. It's now five years of reliving and replaying everything over and over."
She said days before the verdict that if Hillary was found not guilty, she would still believe that he killed her son and nothing could convince her otherwise. (Cyrus appears not to have commented publicly since the verdict was delivered.)
"I don't know that there is a severe enough punishment," she said. "I hope he spends the rest of his life in prison. But he still gets to live."
Crying, Cyrus said she would now tell Hillary, "He took my son. He took a big brother, a grandson, a cousin, a nephew. "[Garrett] was always happy and always willing to help other kids and other people. Never got to go to his sixth-grade graduation. Never got to have his driver's license. No high school graduation. No girlfriend. No marriage. He took his whole life."