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An American teenager has been charged with conspiring to murder her "best friend" after a man she met online offered her $9 million (about R130 million) to commit the crime.

According to investigators, Denali Brehmer, an 18-year-old from Alaska, was recruited to kill her friend by a man a few years her elder, 21-year-old Darin Schilmiller of Indiana.

The pair had previously hooked up online, with Schilmiller assuming a fake identity and posing as a millionaire named "Tyler."

Court documents say that during the course of their online relationship, they discussed a plan to rape and murder someone in Alaska.

Schilmiller allegedly promised Brehmer $9 million or more to send him videos or photographs of the attack.

Brehmer then proceeded to recruit four other teens and the group settled on Cynthia Hoffman - who considered Brehmer to be her best friend - as their victim. 

On June 2, authorities say, 19-year-old Hoffman was lured to a hiking trail north-east of Anchorage where she was bound with duct tape and shot once in the back of the head before being pushed into a river.

Her body was discovered on June 4.

Local news reports said Hoffman's father has described his daughter as having a learning disability and the mindset of a 12-year-old.

Police say the victim was driven to Thunderbird Falls by Brehmer and Kayden McIntosh, a 16-year-old boy, under the guise of going on a riverside hike.

McIntosh allegedly shot Hoffman with Brehmer's gun and dumped her body in the water.

Authorities say Brehmer communicated with Schilmiller throughout the murder, sending him "Snapchat photographs and videos of Hoffman tied up and of the body afterward."

Both Brehmer and McIntosh have been arrested and charged in relation to the murder. Schilmiller has also been arrested along with three others accused of assisting in the planning or execution of the killing.

A grand jury last Friday indicted all six defendants for first-degree murder, conspiracy to commit murder in the first degree, two counts of second-degree murder and other charges.

Schilmiller and Brehmer were also indicted on Tuesday on federal child pornography and child exploitation charges.

Police said that a search on Brehmer's phone during their investigation into Hoffman's death revealed videos of the teen sexually abusing a 15-year-old girl at Schilmiller's behest.

Court documents say Schilmiller admitted to attempting to blackmail Brehmer after the murder into sexually assaulting young girls.

Both face up to life in prison on the child pornography charges.

They also face up to 99 years in prison on each of the other murder-related charges.

Schilmiller had previously tried to befriend other people online through a practice known as catfishing, authorities said, asking his victims to send him photos of their children.  

Speaking at a news conference on Tuesday, Bryan Schroder, the US Attorney in Alaska, warned that the case should serve as a warning to parents.

"For all the good the internet can do, it can be a very dark place," he told reporters. "Parents would be wise to monitor the activity of their children online."

AFP