Facebook is still trying to figure out how the hackers who compromised as many as 50 million accounts used them, including whether they obtained private data or made unauthorized posts.

“This is a really serious security issue,” Mark Zuckerberg, the company’s founder and chief executive officer, told reporters on Friday. While Facebook has already patched the flaw that the cyberattackers exploited, “we need to do more to prevent this from happening in the first place,” he said. “We’re going to keep investing very heavily in security going forward.”