U.S. Customs and Border Protection (CBP) has finalized a measure that asks foreign travelers to provide their social media accounts upon entering the country.

CBP started asking travelers to provide their Facebook, Twitter, and other social media accounts starting Tuesday, Politico reported.

Individuals are prompted to include their social media accounts when applying for a visa waiver.

U.S. Customs and Border Protection introduced the measure in June and took comments on it until August 22, the Guardian reported.

Since the measure was introduced, the government has received much criticism from groups like the Internet Association, which represents Google, Facebook, and Twitter; the American Civil Liberties Union (ACLU); and the Electronic Frontier Foundation (EFF), according to the Hill.

“This program would invade individual privacy and imperil freedom of expression while being ineffective and prohibitively expensive to implement and maintain,” the EFF, ACLU, and 26 other groups wrote in August.

The groups said the measure’s impact would “fall hardest on Arab and Muslim communities, whose usernames, posts, contacts and social networks will be exposed to intense scrutiny.”

The agency has defended the measure, saying it will “identify potential threats” to homeland security. It noted that this is optional, and has previously said foreigners who do not want to provide the information will not be prohibited from entering the country.

Editor’s Note: This piece previously misidentified a CBP officer as a Border Patrol agent. The term has been changed.