Pool photo by Rj Sangosti

After a gunman opened fire on dozens of moviegoers last week in Aurora, Colo., Morgan Jones, an 18-year-old who lives in nearby Denver, tuned into an audio feed of a police scanner, opened a Web browser and began posting every bit of news he could find about the incident on Reddit, the social news Web site. He didn’t know that soon he would be breaking news of his own.

On Reddit, Mr. Jones constructed a comprehensive timeline of the incident as it unraveled, linking to sources of information from Twitter accounts and news outlets and accompanying each post with a time stamp. The live updates quickly gained a following, and soon, another Reddit user invited people who wanted to help contribute to the timeline into a chat room called #theatreshooting on Internet Relay Chat, a global chat system.

By Friday evening, they unearthed something that news outlets seemed to have missed. Reporters had noticed that James Holmes, the 24-year-old-man suspected of the shooting, mysteriously did not appear to have accounts on any of the major social media Web sites like Facebook, Twitter or Linkedin. But Mr. Jones received a private message from one of the chatters containing a link to a profile on a sex-partner seeking site that appeared to belong to Mr. Holmes. Mr. Jones posted a link to the profile on the Reddit timeline, noting that its authenticity was unconfirmed.

The account on Adultfriendfinder.com belonged to someone who used the handle “Classicjimbo.” An examination of the account’s activity reveals that it was created on July 5, and a profile photo was uploaded two weeks ago.

In the photo, Classicjimbo’s hair is red, matching the description of Mr. Holmes provided by police when he was arrested, and his overall appearance resembles photos of Mr. Holmes that were released to the public. A Reddit user even did a pixel-by-pixel comparison of his facial features.

On the site, Classicjimbo’s profile title was updated one week ago to read, “will you visit me in prison?” He says he is single and seeking sexual relationships with a woman or multiple partners.

At about 8 p.m. on Friday, Trinity Phillips, a woman who he listed as a friend on the dating service, and goes by the name “Wynter11,” updated her dating profile to say: “Please do not take it personally if I am hesitant to talk to anyone. I just found out that someone I friended on this site shot 71 people.”

The profile belonging to Classicjimbo was removed from Adultfriendfinder.com at about 8:10 p.m. on Friday. Friend Finder Networks, the company that owns the dating Web site, said it declined to confirm the profile’s authenticity out of respect for its customers’ privacy, but it said that it would assist law enforcement officials.

A federal law enforcement official, who declined to be named because he was not supposed to be discussing an ongoing investigation, said that the FBI was examining the profile but had not concluded it belonged to Mr. Holmes. He said that authorities still needed to cross-check information from Adult Friend Finder with information on the suspect’s computers and phone.

The Reddit users posted a link to the Adultfriendfinder.com profile on the timeline, without concluding whether it actually belonged to Mr. Holmes. Soon afterward, TMZ, The Huffington Post and CNN reported on the sex profile, but without crediting Reddit for its discovery.

“It was sort of a strange feeling, like Reddit’s supposed to be this aggregate news site, but we’re actually breaking news to the media right now,” Mr. Jones said in an interview. “Reddit has so many users and so many people getting online and figuring out what’s going on through their own sleuthing. I think Reddit has some great ability to do that.”

Michael S. Schmidt contributed reporting from Washington.