"Like several EU countries, like the whole EU, Germany is also in the target area of international terrorism. Therefore, I have said for a long time, the situation is serious," de Maiziere told reporters.
He added that there was some indication that the train attacker was from Pakistan rather than Afghanistan, as previously thought, Reuters reported.
The 17-year-old attacked passengers on a train in Bavaria with an ax, severely wounding four Hong Kong residents on the train late on Monday. He then injured a local woman after fleeing, before police shot him dead.
The case is likely to deepen worries about so-called "lone wolf" attacks in Europe and could put political pressure on Merkel, who has welcomed hundreds of thousands of migrants to Germany over the past year.