Strømmer’s body was found by two girls behind a shed in Oslo’s Klemetsrud district in October, having lain there since the summer.
His 21-year-old ex-girlfriend is charged with having attacked Strømner’s upper body with an axe as he lay in bed at her home in nearby Rustadsaga.
Her 22-year-old friend is accused of having helped her to dispose of the body by rolling it in a plastic covering and hiding it near an animal shelter in Klemetsrud.
“The two girlfriends charged with premeditated murder have now admitted they were present when it was carried out and that they planned it for around a week,” prosecutor Kristin Rusdal told newspaper VG.
A text message to a friend on July 9th was the last sign of life from Strømmer.
A ward of the state from the age of five until he turned 23, friends described him as a meek individual who shunned large crowds and occasionally suffered from substance dependency, VG reported in October.
He had told close friends he felt under threat from people he knew and was concerned for his safety.
Police have not released any details about a possible motive for the killing.
The two friends at the centre of the case had considered poisoning Strømner before choosing to use an axe, according to the prosecutor.
Police have also charged a man with helping the two women to chop up the bed in which Strømner was killed. The suspect is believed to have known where the body was hidden.
A third woman has also been charged for failing to inform the authorities of the murder despite knowing about it long before the body was found. All the while, Strømner’s family and friends conducted extensive searches.
Both the most recent suspects had previously been questioned by police but denied any knowledge of the crime.
The case is not expected to go to trial until this summer at the earliest, VG said.