Facebook isn’t the only tech company facing governmental scrutiny over potentially illegal ads. Google and Twitter are also under investigation over their ad platforms, the U.S. Department of Housing and Urban Development confirmed.

HUD hasn’t yet disclosed details of its investigations of Google and Twitter, other than to say that the agency has reached out to both companies in hopes of gaining a better understanding of their ad practices.

Twitter and Google both say they don’t allow discriminatory ads.

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

A Google spokesperson adds that the company prohibits targeting people based on categories including race, ethnicity, religious beliefs, disability status and negative financial standing.

On Thursday, HUD charged Facebook with violating civil rights laws by displaying ads that discriminate based on illegal factors such as race and gender. 

HUD’s complaint against Facebook contains allegations that the company allows advertisers to prevent their ads from being shown to people based on factors like race and gender. The agency also alleges that Facebook’s ad-delivery tool discriminates even when advertisers would prefer to be inclusive.

“Even if an advertiser tries to target an audience that broadly spans protected class groups, Respondent’s ad delivery system will not show the ad to a diverse audience if the system considers users with particular characteristics most likely to engage with the ad,” the complaint alleges.

Industry observers, including government officials, have raised concerns for years that personalized ads can be discriminatory. The White House flagged that concern in a 2014 report on big data.

“The fusion of many different kinds of data, processed in real time, has the power to deliver exactly the right message, product or service to consumers before they even ask,” the report said. “Unfortunately, ‘perfect personalization’ also leaves room for subtle and not-so-subtle forms of discrimination in pricing, services and opportunities.”

Last month, 44 watchdogs — including Public Knowledge, the ACLU, Center for Democracy & Technology and Electronic Privacy Information Center — urged federal lawmakers to include anti-discrimination provisions in any new privacy laws. The organizations should Congress should require companies to “anticipate and protect against discriminatory uses and disparate impacts of big data.”

Facebook recently settled civil rights lawsuits brought by the ACLU, National Fair Housing Alliance and other organizations by agreeing to prohibit advertisers of housing, employment or credit offers from targeting ads based on age, gender, ZIP code and ethnic affinity — often used as a proxy for race. The deal also requires Facebook to pay around $5 million, and create a tool that will allow people to search for all housing ads in the U.S.