The inmate brutally beat warden Karen Gebreab to death as she turned her back to open a door.
While the inmate was quickly convicted of the murder, it has taken longer to determine whether the warden's superiors were partly to blame.
Three people who served as bosses at the prison were prosecuted this summer for breaking work environment regulations through aggravated manslaughter, but have now been acquitted by the Södertörn district court.
According to the court, the prosecutor could not prove that any form of carelessness from the bosses contributed to the murder being able to take place. It could not be proved either that the bosses had neglected their obligations to prevent something like this from occuring, nor could it be proved that the prison lacked sensible routines to prevent it.
The prosecutor, who is still reading the ruling, has not decided yet whether to appeal, according to news agency TT.