CHICAGO (CBS) — It’s supposed to be a place where people can spread the word about crimes, but a man running a Northwest Side neighborhood watch group says he recently caught apparent gang members trying to join.

Now Jorge Muralles is warning others to be vigilant about who they allow into their online block clubs.

Muralles says, last month, people who were flashing gang signs and guns in their social media pictures tried to join the Portage Park Neighborhood Watch Group that he runs on Facebook. He denied their requests.

“We want to keep the neighborhood watch group as a place where people could feel free to say what they have to say, and not feel like they’ll be retaliated against or anything like that,” Muralles said.

A couple days later, concerned citizens started posting in the group about gang taggings in the neighborhood–from private garages to Portage Park itself.

Muralles, who is also a police officer, doesn’t think it’s a coincidence.

“I think they (the gang) were trying to figure out what was going out before they came over here. Kind of trying to get a feel for the neighborhood, I’m assuming,” Muralles said.

His group isn’t the only one that’s dealt with the issue.

The Humboldt Park Neighborhood Watch Page is public, so anyone can join, and the administrator told CBS 2 people flashing gang signs have liked and commented on the page.

In Belmont-Cragin, one neighborhood watch administrator says gang members have even posted bragging about their own gangs or taggings. He quickly deleted the posts.

Muralles’ advice is to closely watch who is trying to join and post if you run an online watch group. If you spot anyone flashing a gang sign, you should deny or delete them. He said, if you know someone in law enforcement, you can always consult with them about whether what you’re seeing is actually a gang sign. Gang members often include gang-related phrases or abbreviations on their own pages, Muralles said.

“It’s important to keep these guys out of the group because otherwise you might have guys keeping an eye out and trying to figure out who’s saying what, who’s doing what,” Muralles said.