A catfish posing as a millionaire duped an Alaskan teen into organising the murder of her best friend, US authorities have alleged.

Key points: Authorities say Darin Schilmiller presented himself as the millionaire Tyler from Kansas, using a fake photograph

Authorities say Darin Schilmiller presented himself as the millionaire Tyler from Kansas, using a fake photograph He allegedly demanded photos or videos of the killing in exchange for millions of dollars

He allegedly demanded photos or videos of the killing in exchange for millions of dollars Denali Brehmer allegedly agreed to commit the murder for him

Denali Brehmer is accused of enlisting four other friends to murder 19-year-old Cynthia Hoffman, who had developmental disabilities.

Prosecutors say the group took Ms Hoffman to a popular hiking spot where she was tied up and shot in the back of the head. Her body was then dumped in a river.

Prosecutors say the catfish offered to pay Ms Brehmer $US9 million ($13 million) if she sent him photos or video of the victim being murdered.

"This is a truly horrific case that is not the norm for our community," Anchorage Police Chief Justin Doll said at a news conference, the Anchorage Daily News reported.

Among those charged is Darin Schilmiller, whom authorities say presented himself as a millionaire called Tyler from Kansas, using a fake photograph.

"He does not look like the young man he portrayed himself to look like, he is not a millionaire and he lives in Indiana," court documents say.

Catfishing is when a person creates a fake identity on a social network account to deceive a specific victim.

Authorities said Mr Schilmiller began an online relationship with 18-year-old Ms Brehmer, of Anchorage.

Denali Brehmer allegedly sent Snapchat videos to Schilmiller throughout the murder. ( Supplied )

About three weeks before Ms Hoffman was killed, the pair began discussing a plan to rape and murder someone, according to court documents.

"Schilmiller offered Brehmer $US9 million or more to carry out the murder and to have photographs and/or videos of the murder sent to him," the documents say.

Ms Brehmer allegedly recruited four friends, including 19-year-old Caleb Leyland, 16-year-old Kayden McIntosh and two other unnamed juveniles, to plan and carry out the murder.

Mr McIntosh, who is being tried as an adult in the case, was the gunman, prosecutors say.

Ms Hoffman was best friends with Ms Brehmer, the documents say.

Officials said there was no indication Ms Hoffman was sexually assaulted.

Snapchat video led to arrest

Ms Brehmer and Mr McIntosh used Mr Leyland's car to take Ms Hoffman on a hike at Thunderbird Falls, a popular location about 32 kilometres north of Anchorage, on June 2.

"Digital evidence and statements show Brehmer was communicating with and sending videos and/or photographs of the events surrounding the incident to Schilmiller at his directive through the duration of the event," documents say.

Kayden McIntosh was enlisted by Brehmer to allegedly commit the murder. ( Supplied )

Two days after the killing, both Ms Brehmer and Mr McIntosh were interviewed.

Mr McIntosh was arrested, but Ms Brehmer denied any involvement in the death.

Police continued to investigate and interviewed her two days later after Snapchat video appeared, in which she appeared to confess, the documents say.

"Brehmer ultimately admitted to being solicited by Schilmiller to commit the murder," the documents say.

Pair also hit with child pornography charges

Mr Schilmiller admitted his role in the plot, saying he chose Ms Hoffman as the victim and he told Ms Brehmer to kill her, according to the court documents.

He also told officials Ms Brehmer communicated with him throughout the murder, and sent Snapchat photographs and videos of Ms Hoffman while bound and then after the murder.

He also allegedly told authorities that he and Ms Brehmer discussed killing another person, but the plan was abandoned, and he admitted to blackmailing Ms Brehmer into raping people.

In a separate federal investigation rising from the investigation, Mr Schilmiller and Ms Brehmer were indicted on federal child pornography charges, including production and coercion and enticement of a minor.

Federal authorities alleged Ms Brehmer produced sexually explicit videos involving a minor and sent them to Mr Schilmiller.

"For all the good the internet can do, it can be a very dark place," Bryan Schroder, the US attorney in Alaska, said at a news conference on Tuesday.

"Parents would be wise to monitor the activity of their children online."

AP/ABC