Image copyright CBS Image caption Family of Cynthia "CeeCee" Hoffman say she had a learning disability

An Alaskan teenager allegedly killed her best friend after an online stranger posing as a tycoon offered her money to carry out the murder.

Denali Brehmer, 18, of Anchorage, is accused of plotting the murder of Cynthia Hoffman, 19, this month.

Ms Brehmer allegedly sent Snapchat video of the killing to an Indiana man, Darin Schilmiller, 21, who had offered her $9m (£7m) for the killing.

Prosecutors said the case should serve as a warning to the parents of teens.

Bryan Schroder, the US Attorney for the District of Alaska, told a news conference on Tuesday: "For all of the good the internet can do, it can be a dark place and parents would be wise to monitor the activity of their children online."

Image copyright CBS Image caption Denali Brehmer

According to court documents, Ms Brehmer confessed to the killing of Ms Hoffman on a hiking trail on 2 June.

Both are described in court documents as "best friends".

The murder was allegedly carried out at the behest of a man she had met online months earlier. He claimed to be "Tyler", a millionaire from Kansas.

But "Tyler" was a bogus online persona concocted by Mr Schilmiller in trap a known as catfishing.

Investigators say they uncovered mobile phone evidence of Ms Brehmer and Mr Schilmiller "discussing a plan to rape and murder someone in Alaska".

Ms Brehmer then allegedly recruited four friends to help in the crime, offering to pay large sums of blood money.

Authorities say Ms Hoffman, who was known as "CeeCee", was brought to Thunderbird Falls in Anchorage by the killers on the day of her death.

They had taken her in a pick-up truck under the guise of going hiking.

Image copyright Flickr/TravelingOtter Image caption Thunderbird Falls

Ms Hoffman's family said she had learning difficulties. The victim's father, Timothy Hoffman, told the Anchorage Daily News: "My daughter trusted these people. My daughter just wanted friends. Now I have to bury her."

The suspects allegedly bound Ms Hoffman's head and hands with duct tape and shot her in the back of the head before pushing her body into the Eklutna River, nearly 30 miles (48km) north of Anchorage.

Kayden McIntosh, 16, is accused of firing the fatal shot with Ms Brehmer's gun.

Caleb Leyland, 19, and a juvenile male and female, who have not been named because of their age, have also been criminally charged.

Image copyright CBS Image caption Police say Kayden McIntosh fired the fatal shot that killed Ms Hoffman

Mr Schilmiller, 4,000 miles (6,400km) away at his home in New Salisbury, Indiana, had also allegedly directed Ms Brehmer to rape the victim before killing her.

But police say there is no evidence that Ms Hoffman was sexually abused.

Prosecutors say Ms Brehmer sent video and photos to Mr Schilmiller in Indiana throughout the attack "at his directive".

The victim's clothing and personal items were allegedly destroyed by the attackers.

Image copyright CBS Image caption Caleb Leyland, 19

Ms Brehmer is also accused of texting the victim's family after the killing to say she had been dropped off at a different park in Anchorage.

Indiana State Police interviewed Mr Schilmiller on 9 June.

They said he admitted duping Ms Brehmer and discussing a second murder plot with her.

He also allegedly blackmailed Ms Brehmer into sexually assaulting children, threatening to expose the murder of Ms Hoffman if she did not comply.

Police say that during their investigation they found sexually explicit images in the pair's text exchanges.

Ms Brehmer has allegedly confessed to filming a video of an eight- or nine-year-old girl being sexually abused, as well as a 15-year-old, and sending them to Mr Schilmiller.

In a criminal complaint, the FBI's Child Exploitation Task Force said text messages show Mr Schilmiller directing Ms Brehmer how she should assault the teenage girl.

He is currently awaiting extradition to Alaska to face the charges.

Each of the defendants faces up to 99 years in prison if found guilty.

In a press conference on Tuesday, FBI Agent Jeffery Peterson issued a stern warning to other would-be criminals.

"If you're listening to my voice, not in Alaska but somewhere else, and you're sitting in your mom's basement, and you're planning to do some type of crime - influence, plan or conduct a crime in Alaska - and you think you're safe because you're that far away, you're not," Mr Peterson said.

"We will track you down. We will find you. And we will bring you back here to face justice."