Washington (AFP) - The FBI is still searching for people who stormed the US Capitol on January 6.

So is Kay.

The 34-year-old from Washington state is one of a number of online sleuths tracking down participants in the attack on Congress.

"We're somewhere between journalists and law enforcement," said Kay, who declined to use her last name for security reasons. "We're dedicated to finding everyone."

More than 725 people have been arrested so far for the attack on the Capitol by supporters of Donald Trump who were seeking to block congressional certification of Joe Biden's presidential election victory.

"January 6 broke my heart and I have never really gotten over it," Kay told AFP, her voice breaking with emotion. "It seemed, like, sacrilegious.

"To me, the Capitol is -- even though I've never been there -- a symbol of our democracy," she said. "And that really matters to me, that we have a healthy, thriving democracy.

"To see the Capitol assaulted like that, and the people inside, was terrifying, just heartbreaking."

Kay has spent months at her computer keyboard, trawling the internet in a hunt for people involved in the assault on Congress.

Attorney General Merrick Garland, in a speech on Wednesday, thanked members of the public for their assistance in bringing participants to justice.

"We have received over 300,000 tips from ordinary citizens, who have been our indispensable partners in this effort," Garland said.

Kay, who has a background in video production, has sifted through thousands of photographs and hundreds of hours of video of the attack available online, much of it on social media.

"I found that people were proudly posting what they had done on January 6," she said. "They were proud to be there, and it was just perfectly logical to brag about it online.

"So you find their social media networks, you know their username and find them across all the platforms that they're on."

'Very satisfying'

Kay is one of a number of online detectives working with groups going by names such as Sedition Hunters, Capitol Hunters and Deep State Dogs.

Kay mainly works with Sedition Hunters, which has a core of about 20 members and the support of hundreds of others.

They use geolocation and facial recognition software such as PimEyes, and appeal for information through Twitter.

The group does not publicly identify suspects by name, only by nicknames, and rigorously verifies photos before publishing them online or passing them on to the FBI.

The FBI, like the attorney general, said the public has been a tremendous help in the investigation.

"The FBI encourages the public to continue to send tips," an FBI spokeswoman told AFP. "As we have seen with dozens of cases so far, the tips matter.

"The FBI is working diligently behind the scenes to follow all investigative leads to verify tips from the public and bring these criminals to justice," she said. 

Devorah Margolin, a senior research fellow at the Program on Extremism at George Washington University, said social media has played an "outsized role" in the events of January 6.

"Most people who participated in these events didn't think that they were doing anything wrong," Margolin said. "They were posting online. A criminal offense took place and they documented it."

Margolin said information obtained online, mostly from social media, featured in 77 percent of 704 January 6 criminal cases researched by the program.

"What we don't know is how much of that information came from citizen detectives or tip lines, and how much was found by the FBI or DOJ themselves," she said.

Aiden Bilyard, 19, was arrested in late November and charged with assault, destruction of government property and entering a restricted building.

In its arrest report, the FBI noted that Bilyard had been identified in open source reporting as #Harvardsweats because of a gray Harvard sweatshirt he was wearing on January 6.

Ronald Loehrke, 30, was arrested in Georgia in early December based in part on photographs published by Sedition Hunters.

Loehrke is charged with assault, obstruction of law enforcement and unlawful entry.

Kay said the work is "very satisfying," particularly when it leads to an arrest.

"Many of these people are very violent people," she said. "That does mean a lot to me, knowing that I've helped get these people off the streets."