By Erin Ailworth

KENOSHA, Wis. -- Worried about escalating violence following the police shooting of a Black man, former Kenosha Alderman Kevin Mathewson on Tuesday posted a call to arms on the Kenosha Guard page on Facebook that he created in June.

Over the two previous nights, Mr. Mathewson said he had seen police outnumbered by crowds attacking officers, looting and setting fires to businesses during protests spurred by the Aug. 23 shooting of Jacob Blake that was caught on a viral 21-second video. Mr. Mathewson, a 36-year-old married father of two who bought his first firearm -- a rifle -- at age 18, wanted to encourage local gun owners to step up that evening.

"I said, 'Are there patriots among us that are willing to take up arms and defend our lives, homes, neighborhoods and businesses?'" he recalled of the Facebook event he posted, which he said began racking up likes. Thousands responded that they would be attending, he said.

It was just one of several similar posts now being scrutinized as police investigate the Tuesday night shooting in downtown Kenosha in which two people were killed and another wounded. Prosecutors have filed charges against 17-year-old Kyle Rittenhouse, an Antioch, Ill., resident whom investigators have identified as the person seen in multiple videos carrying a long gun and shooting at and scuffling with pursuers that night.

Mr. Rittenhouse hasn't entered a plea or commented, and his lawyers haven't responded to a request for comment. Mr. Mathewson said he doesn't know Mr. Rittenhouse or the people the 17-year-old was seen with before the shooting Tuesday night.

It is unclear whether the group Mr. Rittenhouse is seen with in social-media videos was an ad hoc group of armed citizens or something more organized. In a video posted by The Rundown Live, a Milwaukee-based independent news and talk-radio program, the group is described as a militia by the videographer.

In another video by Daily Caller reporter Richard McGinniss that was posted to Twitter, Mr. Rittenhouse explains his presence on the streets of Kenosha on Tuesday night. Mr. McGinniss didn't immediately respond to requests for comment.

"So, people are getting injured...and part of my job is to also help people," Mr. Rittenhouse says in the 19-second clip. "If there's somebody hurt, I'm running into harm's way. That's why I have my rifle because I need to protect myself, obviously, but I also have my med kit."

After the Tuesday shooting, Andy Carvin, a senior fellow at the Atlantic Council think tank's digital forensic research lab, analyzed a number of social-media posts that called for people to take up arms Tuesday night. He said it is often hard to discern where someone stands on the spectrum of militancy and how sympathetic or connected he or she might be to a certain group or ideology.

"On the one end, you may have someone who sees themselves as a gun owner who believes they have a civic duty to help local businesses, and on the other extreme, irresponsible gun owners who are there to knock some heads and get in on the action if the opportunity arises," he said.

Michael German, a former Federal Bureau of Investigation special agent, said another challenge is that militants often take advantage of that ambiguity to obscure their extremism.

"It's true that there are some regular citizens and business owners who think it's necessary to go out and protect my property, but they're not operating in a vacuum," said Mr. German, now a fellow at the Brennan Center for Justice. He said such people need to realize, "They are taking actions that can be associated with the more militant or more racist groups."

Mr. Mathewson said he supports protesters and police reform but remains concerned about the damage done around Kenosha. He added that he was surprised by the attention his Facebook event received. A write-up about it by The Rundown Live was picked up by the right-wing commentator Alex Jones's Infowars website.

While Mr. Mathewson describes the Kenosha Guard as a militia and himself as the commander, he said that there is no formal group and that he was the only person behind the Facebook page, which got little attention until his Tuesday post. He said he thought of the page as a way to get the message out about Second Amendment rights.

"I've never carried a rifle out in public before Tuesday, I promise you that," he said. "In this instance, it made me feel safer."

Mr. Mathewson said when he heard about the shooting the next day he was "sad and disgusted that somebody lost their life." He added: "I think I wouldn't be human if I didn't feel some sort of pause, think...did I have something to do with it?" Mr. Mathewson said. "But I'm pretty confident to say that I didn't have any role."

Mr. Mathewson said that Tuesday's shooting occurred after he was at home for the night, asleep.

Facebook Inc. has designated the Tuesday shooting in Kenosha as a mass murder and removed Mr. Rittenhouse's page from the platform. Facebook also removed the Kenosha Guard page, saying it violated a new company policy about dangerous organizations.

"Doesn't matter if the shooter was a member or not," a Facebook spokeswoman told The Wall Street Journal, adding that it has "found no evidence that suggests the shooter followed the Kenosha Guard Page or that he was invited on the Event Page they organized."

While Facebook didn't take down Mr. Mathewson's call to arms before Tuesday's shooting, the company has since said it was a violation of its rules banning the presence of militias on its platform. In public statements, the company has said that it is still scaling up enforcement of its militia policy and that it should have taken the post down before it did.

Local law-enforcement officials have said they were aware that armed individuals were being encouraged to show up in Kenosha.

At a press conference Wednesday, Kenosha County Sheriff David Beth said he had recently received a call from a person asking whether he might deputize citizens with guns to patrol Kenosha.

"I'm like, 'Oh, hell no,'" Mr. Beth said while recounting the call. Tuesday night's deadly shooting, he added, "was the perfect reason why I wouldn't."

Write to Erin Ailworth at Erin.Ailworth@wsj.com