FBI Director James Comey on Wednesday urged security personnel and the public to be aware of their surroundings and be on alert for signs that someone close to them is being radicalized by foreign terrorists through the Internet — and to report it to police.

Comey made his statements at 1 Police Plaza on Wednesday morning first at the NYPD Shield Conference, which included several hundred private security personnel who work in the private sector and who collaborate with the NYPD, and again at a news conference.

“The threat comes from social media, which revolutionized terrorism,” Comey said.

For example, he said, some Twitter messages cannot be “unlocked” by law enforcement, making it impossible for them track communications between terrorists. Comey said the messages relayed from foreign terrorist groups are as succinct as “I will kill where I am.” Comey said such messages have inspired homegrown terrorists who are receiving these messages on their phones daily.

Comey acknowledged that there is a growing anxiety nationwide about the next terrorist attack.

“There is a tremendous amount of anxiety about the terrorist threats we face today. But we should live in a state of awareness and channel that into something healthy . . . If somebody sees a change, or turn of behavior that is out of line, say something,” Comey said, referring to instances in which it was discovered that family, friends and neighbors ignored signs of radicalization.

NYPD Commissioner William Bratton echoed Comey’s comments. “To prevent crime, disorder and now terrorism we must go where it begins . . . in the minds of those who hate and feel victimized. People who see this are moms and dads.”

Bratton said the terrorists are “propagandizing messages that are slick and professional and are inspiring attacks that we saw in Paris.”