Facebook said on its website Friday that its systems were targeted by hackers in a "sophisticated attack."
The social media site said its security team discovered the breach last month. It occurred when a small group of employees visited a mobile developer website that was compromised, Facebook said. The compromised website allowed malware to be installed on the employees' laptops.
Facebook said that as soon as it became aware of the attack, it repaired the laptops, notified law enforcement and "began a significant investigation that continues to this day."
Facebook said it has no evidence that user information was compromised.