Democratic senators wrote to Facebook CEO Mark Zuckerberg Tuesday, probing the company on whether it has done enough to protect children’s privacy after The Verge reported last month on a flaw in the Messenger Kids app that allowed minors to chat with unapproved adults.

Late last month, Facebook alerted parents of the flaw and notified them that the group chats created by way of it were being shut down by the company. Messenger Kids allows parents to approve of users that their children can speak with in the app. But because of the reported design flaw, some minors were invited to group chats by approved people where unapproved people were also included.

“Children’s privacy and safety online should be Messenger Kids’ top priority,” the senators wrote to Zuckerberg. “Your company has a responsibility to meet its promise to parents that children are not exposed to unapproved contacts, a promise that it appears that Facebook has not fulfilled.”

Facebook did not immediately respond to a request for comment.

In the letter, the senators ask Zuckerberg some basic questions like when the company first became aware of the flaw and how long that flaw existed. They also ask if the company is investigating whether there are other flaws in the app that could violate the Children’s Online Privacy Protection Act, which one of the senators, Ed Markey (D-MA), authored and is looking to update and strengthen in new legislation proposed this year.

The letter requests that Facebook respond by August 27th.