When a mailroom worker at an Amsterdam office building noticed an unusual hissing sound coming from one of the packages being sorted Wednesday morning, he tossed it away. Moments after the parcel left his hands, it exploded.

Less than an hour later, in Kerkrade, a Dutch town near the German border, a second package mailed to a local business blew up. Nobody was wounded in either case and police officials compared the size of Wednesday’s blasts to a small fireworks explosion, saying that the bombs would have caused nonfatal injuries had anyone been holding them.

But the attacks were just the latest in a spate of parcel bombs directed at businesses across the Netherlands that have set the authorities on edge — and they were the first to actually detonate.

The police believe that the latest attacks were most likely linked because the mail bombs were accompanied by letters demanding money in the cryptocurrency Bitcoin to prevent future attacks.