Sony said its PlayStation Network was back online on Monday following a cyber attack that took it down over the weekend.
It was a so-called denial of service attack, whereby the system was overloaded by vast amounts of malicious traffic.
The network itself, and registered information of its 53 million users, was not accessed.
The people who claimed they were behind the attack said it was intended to pressure Sony to spend more of its profits on the network.
A Twitter user with the handle @LizardSquad send the message: “Sony, yet another large company, but they aren’t spending the waves of cash they obtain on their customers’ (PlayStation Network) service. End the greed.”
The same group was also believed to be behind a bomb scare on a commercial airline flight carrying Sony Online Entertainment President John Smedley.
A PlayStation spokeswoman in the United States said the FBI was investigating the diversion of the Dallas/Fort Worth to San Diego flight.
Lizard Squad said it had also targeted the servers of World of Warcraft video gamemakers Blizzard Entertainment, whose website was down for a while at the weekend.
It threatened to attack Microsoft’s Xbox Live network and some users said they had problems accessing it on Sunday.
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