Anyone found guilty would face no more than 20 days in prison, but they'd also be added to an offender registry for the next 10 years.

The last messages were shared in the fall of 2017, but charges are only coming now because it's a "very large and complex case," according to police. The investigation included four Danish police bureaus, and came after US authorities relayed Facebook's warning to Europol.

It can be tricky to catch ad hoc sharing of illegal videos on messaging services like this, both because of the private nature of the messages and because some users are bound to have end-to-end encryption turned on. Ultimately, it relies on someone in the sharing chain deciding to report the offending material -- and in this case, it's clear that it took a while before someone came forward. Privacy concerns are going to rule out active monitoring, but this does suggest that Facebook and others might want to do more to encourage voluntary reports.