An analysis commissioned by the Campaign for Fairer Gambling claims that $4.3 billion worth of illegal and untaxed wagers were placed on the recently completed NCAA men’s and women’s tournaments, accounting for nearly two-thirds of all handle placed on college basketball‘s signature events.

The monitoring, done by YieldSec, showed points where more than 70% of sports wagering-related social media posts and video content were directing audiences toward what it called “illegal betting and gambling operators.” The analysis estimates 43.1 million people — nearly 1 in 7 in the United States — had some sort of interaction with illegal gambling.

“This YieldSec special report illustrates the dire need for a comprehensive strategy to tackle illegal gambling in the U.S.,” said Derek Webb, Founder of the Campaign for Fairer Gambling, in a statement. “The lack of a united government approach and lax oversight by states have only compounded the problem, enabling entities with dubious backgrounds to operate freely. It’s high time for U.S. leadership to spearhead a unified solution to this pervasive issue.”

The CFG estimates that $6.7 billion was wagered on the NCAA tournaments, with only $2.4 billion of that — approximately 36% — being bet legally.

YieldSec’s monitoring, which featured “contextual and relevant search terms for betting, gambling and lottery content,” identified 378 “illegal sports betting operators” targeting the American audience and another 651 affiliates promoting those illegal sports betting operators to U.S. sports bettors. At its peak, 73% of social media content “favored” illegal sports betting operators and 78% of all video content linked to the NCAA Tournaments could be traced to illegal operators.

College sports wagering has been a hot-button issue for sports betting this year. The NCAA is making a concerted push to get states to drop all player-specific prop betting for college sports games, citing player harassment and integrity issues. State agencies in Maryland, Ohio, and Vermont that oversee sports betting placed bans on prop bets prior to the NCAA Tournament, while Louisiana‘s ban on such wagers will take effect Aug. 1.

“The overwhelming presence of illegal gambling during one of the biggest sports betting events of the year is a clear signal that enforcement and monitoring need to be prioritized,” said Ismali Vali, Founder and CEO of YieldSec. “Our findings are a call to action for stakeholders across the board to intensify efforts in combating the spread of unlicensed gambling operations that exploit the online marketplace. With the dominance of illegals across our social media channels, it’s clear that the threats to American commerce, community and consumers are a lot closer to home than ever imagined.”

The American Gaming Association has made previous calls for law enforcement to crack down on illegal betting markets, hoping to get support at a federal level with the Department of Justice, FBI, and U.S. Attorneys’ Offices.