New York CNN Business —  

The Department of Housing and Urban Development has sent letters of inquiry to Twitter (TWTR) and Google (GOOG) about their advertising practices, according to a HUD official.

HUD does not have an open investigation into either of the companies, the official said Thursday. The letters, which were sent late last year, were intended to start a conversation about the companies’ ad practices to better understand how housing opportunities are advertised on those platforms.

The news – first reported by the Washington Post – comes on the same day HUD announced charges against Facebook for violating the Fair Housing Act by “encouraging, enabling, and causing” housing discrimination through its advertising platform.

Twitter told CNN Business it doesn’t allow discriminatory advertising on its platform.

“Someone using Twitter to advertise agrees to comply with the law and not use our services for illegal activities. Discrimination in housing and employment is against the law and against our rules,” a company spokesperson said.

Google said it’s had policies “for many years” that prohibit targeting ads based on “sensitive” categories including race, ethnicity, religion and disability.

“Our policies are designed to protect users and ensure that advertisers are using our platforms in a responsible manner,” the spokesperson added.

Last August, HUD filed a formal complaint against Facebook (FB), alleging the company allows landlords and people selling homes to use its advertising platform to “engage in housing discrimination.” The complaint said advertisers on Facebook (FB) can control who sees housing-related ads based on race, religion, sex, disability and other characteristics.

Facebook said it was surprised by Thursday’s charges as the company had been working with HUD to address its concerns.

Last week, Facebook settled several lawsuits that alleged its advertising platform allowed for discrimination in housing, employment and credit ads. It also outlined several steps to address the problem, including a new advertising portal only for housing, employment and credit ads with fewer targeting options.

Facebook will also roll out a new page for US users that allows them to search for and view current housing-related ads even if they didn’t appear on their News Feed.