BERLIN -- A 17-year-old Afghan asylum seeker who shouted "Allahu akbar" ("God is great") during an ax and knife attack on a train, injuring at least five people, had a hand-painted flag of the Islamic State of Iraq and Syria (ISIS) in his room, a senior German security official said Tuesday.

The attacker attempted to flee, but was shot and killed by a special police unit which happened to be nearby.

"Even during the first emergency call, a witness said that the attacker was shouting 'Allahu akbar' on the train," Joachim Herrmann, Bavaria's interior minister, told ZDF Television. "Also, during the search of his room, a hand-painted IS flag was found."

Herrmann said that it was too early to draw conclusions about the attacker's motive.

ISIS claimed responsibility for the attack on Tuesday, saying the teen was "an ISIS fighter and he carried out the attack in response to the calls to target coalition countries at war with the Islamic State."

The terror group then released a two-minute video purportedly showing the attacker pledging allegiance to ISIS and warning Germany and the Western word at large that ISIS was "now strong and shall target your parliaments" and strike "in the camps, cities, villages, streets, airports, and everywhere else, Allah willing."

The short clip, recorded on a cell phone and seemingly hastily edited, was a far cry from the highly-produced martyrdom videos released by ISIS following previous, larger-scale attacks by ISIS operatives.

In the video, the young man calls on others to "kill these infidels in the countries that you live in." "You can see I have lived in your own home and have planned to behead you in your own territory," the young man says in Pashto while brandishing a knife.

Investigator Lothar Koehler said the teenager's motivation appeared to be Islamic extremism based upon a passage, found among notes in his apartment, which read: "Pray for me that I can take revenge on these infidels and pray for me that I will go to heaven."

Herrmann said, while the Nice attack was "clearly another dimension," in both cases the choices of weapons and targets made them "extremely difficult to prevent in any fashion."

"In one case a truck, in another an ax and knife -- those are the weapons that society cannot logically eliminate, with which any person could equip themselves, which they could put to use at virtually any location at any time of night or day," he said.

Nevertheless, he urged an increased visible police presence across the country.

ISIS has claimed responsibility for several attacks in Europe and the U.S. recently in which it likely played no more than an inspirational role -- the attackers have been found to have no tangible links to the group's leaders or operatives...