No such sticker was found on either of the vehicles, one a stolen mail truck, that Ator used during his rampage, according to Sgt. Oscar Villarreal, a Texas Department of Public Safety spokesman.
Still, the groundless conjecture after the shooting was spread by thousands online and even retweeted by Anthony Shaffer, a former Defense Intelligence Agency officer and a member of President Donald Trump's 2020 campaign advisory board. Shaffer didn't respond to questions about the claim.
The breakneck speed of the misinformation — and just how far it spread — illustrates an eagerness to blame such events on political ideologies, regardless of whether the facts support that. It's also an early indication of how difficult it will be for campaigns to combat virulent falsehoods ahead of a 2020 presidential campaign that could be full of them.
Social media users are unlikely to take the time to research misinformation they encounter online. And even when campaigns try to stamp out potential misinformation, voters might not see or believe the corrections, said Rita Kirk, a communications professor at Southern Methodist University.
"A whole lot of people are just living their lives. They don't have time to go and fact-check a statement," Kirk said. "Truth has been the victim of social media campaigns."
O'Rourke's campaign, which is based in El Paso, home of a mass shooting last month, first received word of misinformation about the candidate early Monday morning from the Democratic National Committee. The campaign then watched the message spread for hours, eventually becoming Google's second-highest trending search query related to O'Rourke in a week.
The DNC monitors for such activity, as well as providing resources for campaigns that want to report or respond to online misinformation as part of the party's larger educational and guidance efforts.
"This kind of thing is a reality that social media has created," said Daniel Wessel, a DNC spokesman. "We all have to stay vigilant in order to combat it."
The first tweet linking O'Rourke to the Odessa shooter appears to have come in shortly after 1 p.m. Central time on Sunday from an account with bot-like tendencies, said Nir Hauser, the chief technology officer for VineSight, which tracks viral misinformation trending online.
Shaffer was among those who retweeted it, adding: "It sounds again like the 'system' failed — Seth Ator had a criminal record and was a @BetoORourke supporter ... this supports my belief that Progressives should be prohibited from owning or having access to weapons — they clearly cannot be trusted with this responsibility."
That image actually came from an online retailer who sells campaign decals. Its owner, Stacy Pyle, said via email that the photo originated from her Etsy store website. But she said she did not sell any version of that decal and had "no clue" the image was being used to spread misinformation.
Unproven statements that the shooter was a "registered Democrat" also spread quickly on social media hours after the rampage. Texas voting records list the 36-year-old Ator registered as an unaffiliated voter in 2012, his most recent record.
Jen O'Malley Dillon, O'Rourke's campaign manager, on Twitter described the bumper sticker claim as a "completely false rumor" but acknowledged that the campaign could do comparatively little to squash it and instead implored social media giants to act.
"These companies claim to be powerless to stop false stories like these from spreading — but their employees are the most sophisticated engineers on the planet, capable of rooting out all kinds of nefarious content," O'Malley Dillon said in a statement to The Associated Press. "They refuse to act on this because they're afraid of the political consequences."
Shaffer didn't answer a message on Twitter, and Trump's reelection campaign did not respond to messages asking if he had specific information on the Odessa shooter being an O'Rourke supporter.
Some mass shootings have had political connections. For example, police say the shooter suspected of killing 22 people at an El Paso Walmart confessed to targeting people of Mexican descent and is believed to have written an anti-Hispanic rant before gunning down mostly Latino shoppers. The same screed reiterated some of Trump's anti-immigrant and anti-Hispanic language.
When 66-year-old James Hodgkinson opened fire on Republican members of Congress during a 2017 baseball practice, police did not provide a motive but said he was "angered by" Trump's election.
In recent months, however, social media users have rushed to mistakenly link shooters and politics in cases where ideology was not believed to play a role.
Some Facebook and Twitter users attempted to suggest a motive for the mass shooting in Dayton, Ohio, last month by highlighting tweets from an account that appeared to belong to the shooter, which bemoaned Trump's election and supported Democratic Massachusetts Sen. Elizabeth Warren, also a presidential candidate.
Authorities have not established a political connection to the shooter's apparent motive, but that didn't stop the posts from spreading or Trump from repeatedly noting the shooter's apparent political leanings.
In June, when 20-year-old Brandon Webber was fatally shot by U.S. marshals during an attempted arrest in Memphis, social media users erroneously circulated a photo of a man wearing a Trump shirt to claim he was the officer responsible for killing Webber. The image was actually a truck driver who frequently posts videos on YouTube. U.S. marshals have not named the officers involved in the incident.
Such inaccurate claims are successful because people want to believe they don't share similarities with a shooter, said Kirk, the communications professor.
"We never want to see somebody that commits those kind of atrocities to be like us," Kirk said. "In this political climate, where there's so much of 'us versus them,' it continues to make a huge divide in our culture."
Seitz reported from Chicago.