KARAK, Jordan — Bullet marks on the thick walls of a Crusader fortress and shattered windows of nearby tourist restaurants — damage from a recent shooting rampage — bear witness to Jordan’s vulnerability to attacks by Islamic extremists.

Some say the assault on Karak Castle by Jordanian followers of the Islamic State group could signal a more aggressive campaign to destabilize the pro-Western kingdom. A senior security official said the Karak attackers had planned multiple attacks in Jordan on New Year’s Eve.