Around 5:45 pm ET, some users began to regain partial access to the three apps.The outage was the second blow to the social media giant in as many days after a whistleblower on Sunday accused the company of repeatedly prioritizing profit over clamping down on hate speech and misinformation.

“To every small and large business, family, and individual who depends on us, I’m sorry,” Facebook Chief Technology Officer Mike Schroepfer tweeted, adding that it “may take some time to get to 100%.”Shares of Facebook, which has nearly 2 billion daily active users, fell 4.9% on Monday, their biggest daily drop since last November, amid a broader selloff in technology stocks.

Facebook services coming back online now – may take some time to get to 100%. To every small and large business, family, and individual who depends on us, I’m sorry.

— Mike Schroepfer (@schrep) October 4, 2021

Shares rose about half a percent in after-hours trade following resumption of service.Security experts said the disruption could be the result of an internal mistake, though sabotage by an insider would be theoretically possible. “Facebook basically locked its keys in its car,” tweeted Jonathan Zittrain, director of Harvard’s Berkman Klein Center for Internet & Society.

Soon after the outage started, Facebook acknowledged users were having trouble accessing its apps but did not provide any specifics about the nature of the problem or say how many users were affected by the outage.

The error message on Facebook’s webpage suggested an error in the Domain Name System (DNS), which allows web addresses to take users to their destinations. A similar outage at cloud company Akamai Technologies Inc took down multiple websites in July.

To the huge community of people and businesses around the world who depend on us: we’re sorry. We’ve been working hard to restore access to our apps and services and are happy to report they are coming back online now. Thank you for bearing with us.

— Facebook (@Facebook) October 4, 2021

Several Facebook employees who declined to be named said that they believed that the outage was caused by an internal routing mistake to an internet domain that was compounded by the failures of internal communication tools and other resources that depend on that same domain in order to work.

Facebook, which is the second largest digital advertising platform in the world, was losing about $545,000 in U.S. ad revenue per hour during the outage, according to estimates from ad measurement firm Standard Media Index.

On Sunday, Frances Haugen, who worked as a product manager on the civic misinformation team at Facebook, revealed that she was the whistleblower who provided documents underpinning a Wall Street Journal investigation and a Senate hearing on Instagram’s harm to teen girls.

Haugen was due to urge the U.S. Congress on Tuesday to regulate the company, which she plans to liken to tobacco companies that for decades denied that smoking damaged health, according to prepared testimony seen by Reuters.