Reddit users have previously been shamed for running their own investigations into criminal matters, but for the Brown University shooting last Sunday one user proved vital for finding the suspect.
On December 13 a gunman killed two Brown University students and wounded nine others in their Engineering building, before escaping by foot.
A five-day manhunt ensued, with police releasing footage of a man believed to be the shooter and asking for the public's help in identifying him.
The suspect evaded detection for days, avoiding facial recognition software by obscuring his face with a medical-type mask and switching the licence plates on his rental cars.
It was only after a tip off from Reddit that police were able to connect the suspect's car to 48-year-old former Brown graduate student Claudio Neves Valente.
On December 18, police traced him to a storage facility in New Hampshire, where they discovered he had taken his own life.
A gun found on Valente was then linked to another murder, that of MIT professor Nuno Loureiro, 47, who was fatally shot in his home in Brookline, Massachusetts, on December 15, two days after the Brown University shooting.
What was the Reddit tip?
On December 15, a man known as "John" posted on Reddit that he recognised the person in the police images of the suspected Brown shooter.
He said police should look into “possibly a rental” grey Nissan, and fellow users responded urging him to contact the FBI.
He said he did, but only met with police on December 17, after approaching them on the street.
The affidavit says John gave investigators additional critical details: he encountered Valente in the bathroom of the engineering building just hours before the attack, where John noted the suspect's clothing was "inappropriate and inadequate for the weather."
John also bumped into Valente outside, mere blocks from the building, where he watched Valente "suddenly" turn around from the Nissan when he saw him.
What ensued was then a "game of cat and mouse," according to John's testimony — where the two would encounter each other and Valente would run away.
At one point, John says he yelled out "Your car is back there, why are you circling the block?"
"The Suspect responded, 'I don't know you from nobody,' then Suspect repeatedly asked, 'Why are you harassing me?'" according to the affidavit.
John told police he eventually saw Valente approach the Nissan sedan once more and decided to walk away.
How the Reddit tip proved vital
His information gave new life to a stalled investigation.
With a known vehicle, Providence police started looking through footage from dozens of AI-powered cameras positioned around the city by the surveillance company Flock Safety.
Flock Safety is a technology company that provides data to thousands of law enforcement agencies across the US, using cameras, licence plate readers and gunshot audio detection.
In the two weeks before the shooting, his vehicle was spotted at least 14 times by the cameras, according to a police affidavit.
Providence police could then ask Flock-using police agencies in nearby cities and states to look for the same car.
New Hampshire police however did not have any images, due to privacy restrictions on how long they can hold images.
Still, without John the tipster — whom local Redditors dubbed "Reddit Guy" — no one would have known how he left.
Previous Reddit smear campaigns
Reddit has previously been criticised for its users running false smear campaigns during criminal investigations.
In 2013 a Brown University student was found dead after taking his own life when Reddit users falsely identified him as a suspect in the deadly Boston marathon bombings
"It definitely went sideways in the Boston Marathon situation," said Michigan State University professor, Liza Potts.
"That's why folks will jokingly refer to the 'Reddit Detective Agency' or the 'Reddit Bureau of Investigations'," she said.
After the most recent shooting Brown University officials sought to swiftly stop another smear campaign circulating on social media platforms falsely tying a current Brown student to the campus shooting.
The "unimaginable nightmare" of false accusations led to "non-stop death threats and hate speech," the student said in a statement.
Concerned that tip lines could be jammed with nonsense, US Senator Sheldon Whitehouse, a Rhode Island Democrat, urged social media speculators to "just shut up".
"There is simply no need from an investigative point of view for people who have no idea what they're talking about to offer their stupid and ill-informed views about what happened all over the internet," Mr Whitehouse said from Congress on Wednesday.
Is Reddit improving its ways?
Ms Potts did say some social media platforms had been working better than others, and "of all the spaces that I study, Reddit seems to be getting it right more than not".
Harmful accusations were largely absent from Reddit's Providence forum, in part because volunteer moderators who manage Reddit's subject matter forums — known as subreddits — are largely responsible for keeping the peace.
Reddit's chief moderator for the Providence subreddit said in an interview that he had been on the platform for about 15 years and remembered the trauma that the false Boston Marathon report caused.
"The Providence subreddit is very sensitive about (not) trying to go on a witch hunt or the mob mentality," he said, speaking on condition of anonymity.
Following the most recent uncovering of the Brown University shooter on Reddit, its user John has received widespread praise.
Providence Mayor Brett Smiley wrote to FBI Director Kash Patel on Friday asking for John to get the entirety of the FBI's $US50,000 ($75,000) reward offered to the public for information on the investigation.
He's "no less than a hero," Mr Smiley said.
Strangers have also invited John to Christmas dinner and suggested he get a "key to the city and free coffee and doughnuts for life", according to fellow contributors to Reddit's Providence forum.
The Associated Press reached out to John, but he did not respond to requests for comment and later posted that he does not plan to talk with media.
AP/ABC
