Five people died during the insurrection, and dozens of arrests are underway nationwide for those who participated in the attack.
Keller, 38, stood out in the raucous crowd because of his towering 6-foot-6 frame and the distinct U.S. Olympic team jacket he had on.
The jacket had “USA” printed across the back and down the sleeves, which former team members immediately recognized, The New York Times reported.
Keller’s identity was also exposed because his green face covering was lowered around his neck.
A video posted by a reporter from conservative news outlet Townhall appeared to show Keller among rioters pushing and shoving police officers who were trying to clear the Capitol Rotunda, according to the criminal complaint.
Keller’s alleged attendance at the riot was first reported Monday by the swimming news website SwimSwam, but video placing the former Olympian in Washington had been circulating online since late last week.
Several people who recognized Keller in the video reported him to authorities, the Times said.
Keller has also deleted his social media accounts, where his content and messaging showed him to be an avid Trump supporter, according to those who followed him.
The man thought to be Keller in the video was not shown committing any acts of violence during the siege, reports said.
If authorities confirm that Keller entered the building, he could face federal charges that include unlawful entry and disorderly conduct. More serious charges could include theft of government property and firearms violations, which numerous other rioters now face.
Keller was a member of three U.S. Olympic teams, and he won medals at three Olympics, including golds at the Athens Games in 2004 and at the Beijing Games in 2008 as a member of the 4x200-meter relay team. He won silver in the event at the 2000 Sydney Games.
A powerful freestyler, Keller produced his signature moment in 2004 in Athens. Days after winning a second straight bronze medal in the 400-meter freestyle, Keller gained a measure of revenge when he held off the gold medalist, Ian Thorpe of Australia, on the anchor leg of the relay final. Phelps swam the leadoff leg for the American team, which included Ryan Lochte and Peter Vanderkaay.
Keller went on to become a real estate agent in Colorado Springs, where he runs Hoff & Leigh, the Times reported. On Monday night, the company removed Keller’s profile and nearly all references to him from its website.
A cached version of his Hoff & Leigh contact page included a biography that described Keller as a broker associate with three years of real estate experience. His career as an elite athlete, the company said, had made him “extremely driven.”
Asked about Keller’s employment, a woman who answered the phone at the company on Tuesday said, “We can’t give out any information on that at this moment.” A call to a cellphone number for Keller returned a message that he was not available.
Information provided by The New York Times was used to supplement this report.