BERLIN -- A 17-year-old Afghan asylum seeker who went on a rampage on a Bavarian train with an axe and a knife appears to have acted alone, goaded on by online Islamic extremist propaganda, Germany's top security official said Wednesday.

Five people were injured in the attack and two remain in critical condition, "and it is not yet clear if they will all survive," Interior Minister Thomas de Maiziere told reporters in Berlin.

The Islamic State group has claimed responsibility for the attack and though German authorities have said that they have found no direct link to the extremists, an IS flag was found among the suspect's possessions and he refers to himself as a "soldier of the Islamic State" in a video that was posted by the group.

De Maiziere said it was not yet clear when the video was made, but confirmed that investigators had determined it was authentic. It shows the young man waving a knife as he talks into the camera.

"It appears to be a classic farewell video of a suicide attacker," de Maiziere said.

In the video, the suspect, whose name has not been released, urges others to commit attacks, saying "if you can't come to Iraq and Syria the least you can do is kill these infidels in the countries that you live in."

In the video the suspect speaks in Pashto, one of Afghanistan's main languages, with an accent that suggests he is from the eastern part of the country. The accent is also similar to that of Pakistanis who speak Pashto, leading to speculation that he may have lied about his country of origin when he came to Germany last year as an unaccompanied minor asylum-seeker, to increase his chances of being allowed to stay.

De Maiziere said authorities were looking into the possibility that he might actually have been from Pakistan, but that other evidence speaks to his being from Afghanistan, including comments he made about a friend in Afghanistan having recently been killed -- something authorities think may have prompted him to plan his attack.

German authorities also have statements about family from Afghanistan on immigration documents, de Maiziere said.

"There needs to be further investigation," he added.