Duped by a man she met on the internet who lived nearly 4,000 miles away, an 18-year-old Alaska woman lured her best friend to a popular trailhead and carried out a murder-for-hire plot on the promise of millions, the authorities say.

The woman, Denali Brehmer, even took Snapchat photos and videos of the homicide as proof, according to court documents filed Friday.

Ms. Brehmer has been charged with first-degree and second-degree murder and a raft of other criminal counts in the slaying of Cynthia Hoffman, 19, who went for a hike June 2 on the outskirts of Anchorage, where she lived, but never returned.

Two days later, investigators recovered Ms. Hoffman’s body on a bank of the Eklutna River near Thunderbird Falls. She had been bound with duct tape and shot in the back of the head, the authorities said.