Its head, Anders Thornberg, says "we have never seen anything like it" and propaganda from the Islamic State group was a key factor.

He says the figure was in the hundreds a few years ago, adding it was "a historic challenge with extremist environments growing."

Thornberg told Sweden's news agency TT Friday that SAPO gets about 6,000 pieces of intelligence every month, up from 2,000 five years ago. He didn't go into specifics.

Sweden's threat assessment remains three on a five-level scale. On April 7, the driver of stolen truck killed five pedestrians and injured 14 in central Stockholm.