"As we know in Utah, the weather changes very frequently," Rivera said, later adding that the weather is "very unpredictable."
About an hour later, the brother reached the east peak first and turned around to head back down after noticing the weather had worsened, Rivera said. Nelson continued toward the summit. The brother told investigators that he saw Nelson suddenly collapse about 20 feet away from him and believed he had been struck by lightning.
The brother rushed back toward the Snowbird tram until he regained cellphone service and called 911 shortly after noon.
A Utah Department of Public Safety helicopter quickly located Nelson near the summit, but rescuers had to wait for the storm to move through before they could safely reach him. He was pronounced dead at the scene. Snowbird Ski Patrol later helped escort Nelson's brother off the mountain, according to the Salt Lake County Sheriff's Search and Rescue team.
American Fork Twin Peaks rises to 11,461 feet and is the highest point in Salt Lake County. Rivera emphasized that mountain weather can change with little warning and urged hikers to check the forecast before heading out. She said hikers should leave exposed ridgelines and summits as soon as storms begin moving in.
The sheriff's office also urged hikers to move to lower ground, stay away from isolated trees and bodies of water, and, if they cannot reach shelter, crouch low with their feet together until the storm passes.
The Salt Lake County Sheriff's Office said the investigation remains open while the Utah medical examiner determines Nelson's official cause of death.