Police have found no potential accomplices or any other individuals who may have directly encouraged Anton Lundin Pettersson to carry out the killings.
"There are no indications of that," Sten-Rune Timmersjö, investigator with Trollhättan police, told Swedish Radio News.
Lundin Pettersson entered the Kronan school wearing a mask on October 22 last year. He attacked people who appeared to have an immigrant background with a sword.
The report does not provide any indication as to what drove Lundin Pettersson to the attack.
The police had been criticised after it emerged that no interviews were conducted with people that Lundin Pettersson had been in contact with over the internet.
Subsequently, officers specialising in criminal and psychological profiling have been involved in the investigation.
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