Facebook and Instagram suffered outages on Monday evening that left users unable to access the sites for almost an hour. Hacker group the Lizard Squad has claimed responsibility for the attack.
However, Facebook said in a statement that the outages were not the result of a hack, but of an in-house error.
"Earlier this evening many people had trouble accessing Facebook and Instagram," the firm said.
"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 percent for everyone."
Despite reassurances from Facebook, the Lizard Squad said on Twitter that it was responsible. It also claimed to have affected other web services Tinder, AIM and HipChat.
The group has hit the headlines several times in the past few weeks, most recently after hacking into the website for Malaysia Airlines on Monday.
Professor Alan Woodward of the University of Surrey said that the Facebook incident underlined the difficulty of knowing who to believe when claim and counter-claim take place.
"Facebook has confirmed that the downtime was caused by an engineering mistake. However, the group called Lizard Squad had only to mention the problem on Twitter and it caused instant speculation that it was they who had caused the problem," he said.
"In the same way that it is very difficult for law enforcement agencies to attribute blame for cyber attacks, it is very easy for hacking groups to plausibly claim any significant online incident as their work."
Professor Woodward said that it was clear from Facebook's swift rebuttal of the hacking claims that firms are becoming aware of the need to make users confident that the sites they visit are not easy targets for hackers.
"It is a sign of the nervousness caused by the growing number of successful cyber attacks on major online brands that Facebook quickly issued a denial that the incident was a cyber attack," he added.
"It's better to admit to a cock-up by their own staff than blame it on an outside group."
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