In many of these cases, police say the people they arrested actually talked about what their plans were. Some said they were just joking. But JJ Klaver, who spent 26 years with the FBI in Philadelphia, said anything perceived to be a threat is no laughing matter these days.
"Obviously there's this message if you see something or hear something, it should be reported to law enforcement. So I think it increases the public's awareness of the importance of red flags," he said.
That's what happened in one of the most recent events. Police in California arrested a Marriott hotel cook who allegedly threatened to shoot co-workers and guests. A co-worker called police.
"They're members of the community," Klaver said. "They're at work, they're at school. So there are people who are in contact with them every day, in direct contact with them. So we all need to be alert and aware of what potential warning signs are."
The man, 37-year-old Rodolfo Montoya, was arrested, and police seized firearms, including an assault rifle, and hundreds of rounds of ammunition from his home.
There's also the vast world of social media, where Klaver said "it is nearly impossible to monitor everything."
In one recent incident, police arrested a Tennessee man who they say threatened to shoot up a Planned Parenthood in Washington, D.C. However, he posted the alleged plans on the social media site iFunny, which is known for white supremacist content.
While law enforcement have tools in place to stay ahead, Klaver said it's also important for people not to hesitate and to report alarming behavior they see online.
"With these incidents happening now, at least in the near term, there might be more people talking about these type of incidents in a copycat fashion," added Klaver, "but you also have much greater public awareness."