BERLIN—Seventeen operatives of Islamic State have entered Germany disguised as refugees, according to evidence collected by the country’s domestic intelligence agency.

Most of the group are in prison or dead, Hans-Georg Maassen, the head of Germany’s Federal Office for the Protection of the Constitution, said Saturday in an interview with daily Frankfurter Allgemeine Zeitung. The agency, he said, has tracked more than 320 attempted contacts between Islamists and refugees in the country.

...