According to the latest stage of investigations into the Würzburg axe attack, the assailant did not receive orders from Isis to carry out the rampage.
Interior Minister Thomas de Maizière said on Wednesday that the adolescent had been incited by Isis propaganda, but that this didn't make him a member of the group.
However federal prosecutors, who have taken over the investigation, said in a statement they would probe the extent of the teenager's links with Isis.
The video which Isis released on Tuesday does not contain any evidence the jihadist organization had ordered him to attack civilians, de Maizière said. At the same time the Interior Minister confirmed the authenticity of the video featuring the teen identified in media reports as Riaz Khan Ahmadzai.
“It is perhaps an incident which falls somewhere between an Amoklauf (lone rampage) and an act of terrorism,” de Maizière said.
Germans used the term Amoklauf to refer to acts of violence which tend to be carried out by psychologically disturbed people and the motive for which is hard to define.
The Interior Minister spoke of the attack, which left four people seriously injured after the assailant attacked them with an axe on a train in northern Bavaria on Monday evening, as “a brutal act of indiscriminate violence.”
De Maizière also said he saw no reason to doubt that the attacker was from Afghanistan, despite earlier suggestions that the 17-year-old may have lied about his identity.
He noted that both Afghanistan and Pakistan had Pashtun communities and said German authorities had received an application for family reunification from the teenager for relatives in Afghanistan.
He added that investigators were also looking closely at accounts from the assailant's foster family that he received a phone call Saturday informing him of the death of a friend in Afghanistan and whether his apparent distress over the news was a possible trigger for the attack.
Sources close to the German security services said earlier on Wednesday they think he might have pretended to be Afghan on arrival in Germany in 2015 in order to have a better chance at securing asylum, television station ZDF reported.
“The background to the attack needs to be investigated further,” the Interior Minister cautioned, but he spoke in favour of more CCTV, a stronger police presence and more protection for officers.
Authorities are doing everything they can to stop terrorism, but there is never a guarantee it will succeed, he said.
De Maiziere said there have been “leads on connections to international terrorism” among refugees but that in most cases these had proved insubstantial.
Some cases had yet to be fully investigated, he added. “For this reason we cannot say that there is no connection between refugees and terrorism.”