WASHINGTON (AP) — Executives of Facebook, Google and Twitter faced questioning by a House panel Wednesday on their efforts to stanch terrorist content and viral misinformation on their social media platforms. The scrutiny comes as the tech giants step up safety measures to prevent disinformation online targeting the Democratic presidential debates.

Lawmakers and tech industry executives are concerned that the debates could be targeted by Russian or other hostile parties to foment political conflict online, as happened in the 2016 election.

The hearing by the Homeland Security Committee was prompted by the mosque shootings in New Zealand in March that killed 50 people, attributed to a self-professed white supremacist who livestreamed the attacks on Facebook.

Committee Chairman Rep. Bennie Thompson asked what the companies are doing to keep hate speech off their platforms.