Three weeks after the riot at the U.S. Capitol, a posting appeared on the social media site Nextdoor with the heading, “White Truck — Suspicious Behavior?”

“There’s this suspicious white truck that is parked on the corner of Belmont and 19th,” one person wrote on the hyperlocal network. The writer said the driver was a middle-aged White man who appeared to do nothing except sit in the truck all day. The license plates said, “Don’t Tread on Me.”

Follow-ups by the writer and others said the truck was registered in Virginia and had been parked at the same spot in Adams Morgan in Northwest Washington for several days.

“Doesn’t appear he’s doing work in the area, just sitting in the car watching people,” an entry read.

Some members immediately recommended alerting police, perhaps through a non-emergency line, saying it was better to be safe than sorry. Several mentioned the storming of the Capitol and the Christmas morning RV explosion in Nashville as sufficient justification to alert D.C. police or the FBI.

As details were added or revised, the thread picked up momentum and urgency. Commenters also began to question or criticize others, and side disputes broke out over accusations of sexism and racial profiling.

“It kinda turned into a dumpster fire with just all the comments, the snide remarks,” Michael Landry, 50, a carpenter, said in an interview after posting in the thread. “I’m actually kind of ashamed of some of the things I said — but I stand my ground on everything I said.”

Days later, after racking up as many as 102 comments and 35 emoji, the thread petered out. But it captured a moment when the nation’s capital — under siege from a pandemic and the threat of political violence — was on edge.

D.C.′s 911 center has reported a 20 percent increase in calls regarding suspicious activity since a mob supporting President Donald Trump stormed the Capitol last month. The Office of Unified Communications logged 1,624 calls (suspicious vehicle, package, etc.) since Jan. 6, compared with 1,353 calls in the same period a year earlier, spokeswoman Wanda Royster Gattison said.

Social media has become an important tool in monitoring extremist threats. Law enforcement officials routinely gather information on sites where violent extremists congregate, and members of the public have flagged ominous Twitter and Facebook posts to avert danger.

Nextdoor, which has spread to 11 countries and 1 in 4 U.S. households since its launch in 2011, combines features of listservs and social media such as Facebook or Craigslist.

Members, who are grouped by their immediate neighborhoods, use the app to report bird sightings and alert others about missing pets. It allows them to list items for sale or warn neighbors about criminal activity. As the pandemic took hold last year, membership almost doubled as people used the app to share acts of neighborly kindness.

But it’s also been called a site for tattletales and people obsessed with package pirates. A Twitter feed called “Best of Nextdoor” that highlights its funny or absurd postings has gained more than 427,000 followers.

Some blame the social network for increased racial tensions and unnecessary police intervention, citing what they say is profiling by users. In recent years, the San Francisco-based company has taken steps to inhibit racial profiling while gearing the network toward advertising local businesses.

Nextdoor spokeswoman Edie Campbell-Urban said in an email that the company connects people in meaningful ways that also allow them to turn to each other in times of crisis or tension. But the company also has guidelines that require members to be considerate of others.

“We have very strict Community Guidelines in place that explicitly state that we don’t allow profiling or shaming of any kind,” she wrote. “Every neighbor on Nextdoor has a responsibility to act in accordance to those guidelines, and participate in conversations that are civil and respectful. If conversations fall outside of our guidelines, members have the tools to report the content for review and if found to violate our guidelines, we will remove the content.”

As the thread about the white truck unspooled last month, people imagined potentially dangerous scenarios, such as a Nashville bombing copycat. Others, picking up on a Department of Homeland Security advisory that day about the domestic threat from “ideologically-motivated violent extremists,” suggested that the truck fit the profile of an insurrectionist’s vehicle or worse.

“The Don’t Tread on Me tags say he probably has guns and is probably angry at some minority or politician,” a person wrote in reference to the truck’s license plates, whose theme is borrowed from the Gadsden flag, a symbol dating to the American Revolution and variously adopted by libertarians, the tea party and the far right.

Eric Olsson raised concerns in a posting that the truck posed a threat to former president Barack Obama, who lives about a half-mile away in the Kalorama neighborhood. In an interview, Olsson said he thought its driver might have been waiting for an opportune moment to drive a bomb past Obama’s Secret Service detail.

“You know how it is [that] the more you think about it the worse it gets?” Olsson said.

Like others, Olsson said his thinking had been shaped by the riot at the Capitol, especially because many participants remained at large and no one was arrested for planting pipe bombs at Republican and Democratic party headquarters on Capitol Hill.

“I just want the police to be aware of it,” said Olsson, 52, a professional translator. “It doesn’t mean I want everybody arrested who’s doing something suspicious.”

Another commenter theorized that the truck might be involved in sex trafficking. Somebody else wanted to know whether there had been reluctance to alert police because the driver was White.

“I was a little bit freaked out, to be honest,” said Allison Somogyi, 33, who moved to the District three years ago after completing a PhD in history. “Most of the time, my reaction to stuff on Nextdoor is that people are overreacting. Usually, it’s people complaining about stuff that’s not important. But we’re really nervous after what happened with the insurrection downtown and what happened in Nashville.”

Others questioned the thread, with at least one person saying the initial poster should have just approached the driver herself to find out what he was doing.

“That was mostly men, I think,” Somogyi said later. “Because I don’t think any woman would be like, ‘Yeah, go up to this random strange man.’ ”

Landry, in his posting, showed no patience for those who were not taking the potential threat seriously, telling one person on Nextdoor to “shut your mouth.”

“When things are serious, things need to be taken seriously — instead of just being a sheep. You got to look out for your tribe,” Landry said later. Since the storming of the Capitol, he said he’s been thankful for other people’s vigilance because he’s been extra alert, too, for out-of-town license plates, bumper stickers and people who aren’t wearing masks against the novel coronavirus.

But the thread also included pushback from some who thought it alarmist, wildly speculative or prejudicial.

“Seriously? No one has ever seen an illegally double-parked vehicle with a contractor or someone else inside slacking off for hours?” Adrian Constantyn wrote. “You clearly haven’t lived in DC long enough.”

In an interview, Constantyn said the thread reminded him of the days after the Sept. 11 terrorist attacks when anyone wearing a turban might be reported as a terrorist — and perhaps even the false claims of election fraud that motivated the pro-Trump mob to storm the Capitol.

“I thought the thread went off the rails, so I extracted myself from it,” Constantyn, 47, said. “And it’s a cautionary tale about how social media can get out of hand.”

In the end, D.C. police received one notification about the truck, a department spokeswoman said. Characterized as a “parking complaint,” the request for service came in at 10:13 a.m. on the day the initial Nextdoor posting went up.

Twenty-one minutes later, an officer responded and determined that the unidentified driver had been working on a construction project. No police report was filed.