A bomb exploded!
It's not the threat of a terrorist act that gets NCIS on the move during NCIS: Los Angeles Season 5 Episode 23. There was an actual bombing at Venice Beach that killed two people and injured many others, including children.
Before the team even began their investigation, ZNN Reporter Dana Steele broadcast that the JLF were responsible for the bombing. The organization hadn't claimed responsibility for it, but she reported it based on information from her source. She thwarted NCIS's investigation by sending them after a fake source.