The 18-year-old from Anchorage, Alaska, had developed an online relationship with Darin Schilmiller, 21, of IN, who posed as a millionaire named "Tyler", and offered her at least $9 million to kill Cynthia Hoffman and send him "videos and photographs of the murder", according to the Alaska Department of Law.
Brehmer was supposed to send photos and videos of the killing back to Schilmiller, according to the court documents.
According to the new filings, Brehmer enlisted four friends - identified as Kayden McIntosh, 16; Caleb Leyland, 19; and two other teens whose names are being withheld due to their ages - to help plan and carry out the murder. When later questioned by police over Hoffman's death, Brehmer allegedly confessed to her role in the murder plot.
On June 2, Brehmer drove McIntosh and Hoffman to Thunderbird Falls under the guise of going on a hike, the documents said, citing multiple statements and other corroborating physical evidence.
Once the group stopped at a clearing, Hoffman's hands, feet, head, and mouth were bound by duct tape, and McIntosh shot her once in the back of the head with Brehmer's gun. Authorities found the teen's remains on a river bank on June 4.
Brehmer told police she shot video of a girl who was 8 or 9 and another who is 15 and sent them to Schilmiller.
Timothy Hoffman, the victim's bereaved father, said his daughter had only been friends with Brehmer for about half a year and believes she took advantage of her deep desire to form social bonds and be accepted, according to Anchorage Daily News.
Prosecutors say McIntosh then used Brehmer's gun to shoot her through the head before dumping the body. Brehmer claimed the man also directed her to assault a young child and a 15-year-old.
Brehmer and McIntosh face up to five years imprisonment for each of the tampering with physical evidence charges.
Mr Schilmiller is in the custody of the Federal Bureau of Investigation and will not have contact with his co-defendants.
In a press conference Tuesday, Bryan Schroder, U.S. attorney for the state of Alaska, announced Schilmiller and Brehmer have also been indicted by a grand jury on federal child pornography charges.
Hoffman said he didn't know about the Grunwald case until he met Grunwald Tuesday. The duo is now facing charges of conspiracy to produce child porn, production of child porn, receipt and distribution of child porn and coercion and enticement of a minor. He was charged with first-degree murder and conspiracy to commit murder.
Phone records show Brehmer was sending videos and photographs to Schilmiller "at his directive" throughout.
Brehmer, McIntosh and Leyland are now behind held by the Department of Corrections and will be arraigned on Tuesday, the department added. They sent texts to the victim's family members via her phone. It has a policy of not commenting on pending cases. He also admitted to telling Brehmer to kill Hoffman and that he and Brehmer were planning a murder for three weeks.
The two had even planned a second killing, but the plan was "ultimately abandoned", Schilmiller told investigators as he admitted his part in the crimes, according to the documents. Cynthia had such a kind heart and was a friend to many people.