WASHINGTON (NEXSTAR) — A website detailing the infrastructure of the underground passageways in Washington, D.C., suddenly became quite popular ahead of the Capitol insurrection on Jan. 6.

Elliot Carter, the D.C. history buff and writer who operates the site, recently told NBC4 Washington that traffic to WashingtonTunnels.com started to spike on New Year’s Day after users from anonymous message boards began sharing the link among themselves.

Carter tipped off the FBI out of concern, he said, after tracing some of the traffic back to forums named for militias, weapons or Donald Trump.

WashingtonTunnels.com, meanwhile, contains no information or maps that aren’t available to the public elsewhere, or already published on government-run websites. But the FBI took Carter’s tip seriously enough, as evidenced by a Senate report released in the spring concerning the events of Jan. 6.

According to the report, Deputy Chief Sean Gallagher of the U.S. Capitol Police’s Protective Services Bureau had confirmed he was made aware of “significant uptick” in the website’s traffic, via the FBI National Threat Operations Center.

The report also noted that an online forum driving traffic to Carter’s site was “promoting photos of the tunnel system and stating their goal of finding Democratic Members early to block them from entering the Capitol,” according to the tip the FBI received.

A representative for the U.S. Capitol Police, however, says that the USCP and other law enforcement officials were already planning to enhance security at the Capitol after receiving multiple tips — not just Carter’s — as well as reports from federal intelligence experts.

Despite the information, the USCP maintains its intelligence did not indicate a planned “large-scale attack” on the Capitol.

“A law enforcement source alerted U.S. Capitol Police leadership to the spike in website traffic regarding the maps,” the U.S. Capitol Police wrote in a statement. “Ahead of January 6, the USCP knew Congress and the Capitol grounds were to be the focus of a large demonstration attracting various groups, including some encouraging violence. Based on this information, the Department enhanced its security posture. What the intelligence didn’t reveal was the large-scale demonstration would become a large-scale attack on the Capitol Building as there was no specific, credible intelligence about such an attack. The USCP consumes intelligence from every federal agency. At no point prior to January 6 did it receive actionable intelligence about a large-scale attack.”

Carter told NBC4 he temporarily removed his website from the internet after the insurrection, though it’s currently back online.

“Given their importance to daily life in the nation’s capital, it’s surprising to find that the full picture of Washington’s various tunnels remains unpainted,” he explains on his personal website. “I’m currently working to fill that gap.”