Fox reported they were arrested and being questioned about "suspicious cell phone usage", that seems to indicate that there was some seriousness to the situation to me!
Nope...according to Dutch sources...they are being released...and have nothing to do with terror....
You'll know it was serious as soon as the FBI rules out any link to terrorism.
Just the fact they released the names shows that it was a serious situation. If it were all a misunderstanding and no real harm was intended why would they release the names? You usually release a name before they are going to be charged with something. If it is just a misunderstanding the authorities usually try to protect their privacy.