The social networking site Facebook became inaccessible across much of the world this morning.
Millions of users were unable to access their accounts, with users in some countries also having difficulties accessing photo-sharing app Instagram. Dating app Tinder - which relies on Facebook to use its service - was also affected by the problem.
The sites appeared to have been unavailable for 40 minutes before coming back online.
Facebook believed its own engineers had caused the problem, dismissing claims that a hacking group had been responsible.
A spokeswoman for Facebook said: "Earlier this evening many people had trouble accessing Facebook and Instagram.
"This was not the result of a third-party attack but instead occurred after we introduced a change that affected our configuration systems.
"We moved quickly to fix the problem, and both services are back to 100% for everyone."
The hacker group Lizard Squad had tweeted about the services going offline, leading to reports that it might have been responsible. Lizard Squad has been blamed for attacks at the end of last year that forced Sony's PlayStation network and Microsoft's Xbox Live gaming services offline.
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