[Atocep]

Originally Posted by Edward64

Where does the article say that? Did he tell a co-worker about it or did the co-worker assume something and reported him?

Sure, if a co-worker came in and said "I'm going to shoot you/everyone", pretty good grounds for arrest. All the article said was "a tip" with no other details.

Disgruntled cook threatened mass shooting at Long Beach Marriott - Los Angeles Times

Police said Montoya told a coworker Monday that he planned to come into work and shoot everybody he saw at the hotel.