With concerns raised over Backpage.com’s role in facilitating sex trafficking, Mastercard and Visa have stopped doing business with the classified ad site and users will no longer be able to use the credit cards to place ads.

Their decisions come days after the two companies received letters from Cook County, Illinois Sheriff Thomas J. Dart urging the Mastercard and Visa to cease their credit cards’ transactions on the site.

“The use of credit cards in this violent industry implies an undeserved credibility and sense of normalcy to such illicit transactions and only serves to increase demand,” Dart wrote in the letters sent Monday.

Mastercard said Tuesday that it was cutting ties with the website, with a spokesperson confirming to TPM that it approached Backpage’s bank and that bank is no longer process Mastercard payments for Backpage.com transactions. Visa sent TPM a statement Wednesday that it had taken the steps to stop processing Backpage.com payments.

Mastercard’s decision was first reported by the Wall Street Journal.

Dart’s letters were part of larger campaign Cook County planned to launch publicly Wednesday pressuring the credit card companies to reassess their relationship with Backpage, as critics say the site has not done enough to curb trafficking among its ads. Craigslist ended its adult services section in 2010 due to similar concerns.

Cook County, which includes Chicago, is the second largest county in the country and is the site of 20,000 Backpage prostitution ads, according to the letter.

“[T]he unfettered proliferation of websites like Backpage.com has provided this violent industry with a mask of normalcy, driving demand ever higher and increasing the enslavement of prostituted individuals, including children,” Dart wrote, alleging that in more than 800 cases where his office responded to Backpage ads, every investigation led to an arrest.

American Express already does not do business with Backpage.

Backpage vehemently denies that its adult pages have become a hub for sex trafficking.

“We do more, to my knowledge, than any other online service provider to try to prevent the use of our service [for trafficking minors],” Backpage’s general counsel Elizabeth McDougall said in 2014. The company has argued that if it ended its adult section, it would push that advertising content to darker places on the web where it would be even harder to monitor for criminal activity.

McDougall declined TPM’s request for comment on Mastercard’s decision.

For years, anti-human trafficking advocates have targeted Backpage for its adult content. In 2012, a hedge fund run by Goldman Sachs sold its stake in Village Voice Media, Backpage’s parent company, under pressure from the likes of New York Times columnist Nicholas Kristof and the feminist blog Jezebel.

Dart’s campaign has raised the concerns of some First Amendment advocates, with Electronic Frontier Foundation activism director Rainey Reitman telling USA Today, “We shouldn’t have informal pressure from public officials forcing financial service companies into deciding which types of speech should and shouldn’t be allowed.”

Nevertheless, anti-trafficking groups have taken notice of the approach.

“Relying on Backpage’s internal measures is, in our view, not stopping the sex trafficking on the site, so we are supportive of additional efforts,” Bradley Myles, the CEO of the Polaris Project, said in an interview. “It is a powerful new tool to address trafficking, if traffickers can’t pay to place an ad for a child or they can’t pay to place an ad for an adult woman they’re controlling because the major credit cards won’t allow it.”

Update: This story has been updated to reflect Visa’s decision Wednesday to end its relationship with Backpage.