* Sony's PlayStation Network back up after attack
* No information accessed or stolen - Sony
* Video game maker Blizzard Entertainment's site still down (Adds comments from FBI spokesman Joshua Campbell and cybersecurity expert Daniel Clemens)
By Malathi Nayak and Sophie Knight
SAN FRANCISCO/TOKYO, Aug 25 Sony Corp's PlayStation Network was back online on Monday following a cyber attack that took it down over the weekend, which coincided with a bomb scare on a U.S. commercial flight carrying a top Sony executive.
Sony said on its PlayStation blog that the network had been taken down by a denial-of-service-style attack, which overwhelmed the system with traffic but did not intrude into the network or access information of its 53 million users.
Meanwhile, the Federal Bureau of Investigation (FBI) said it was investigating the bomb threat that caused an American Airlines flight to be diverted.
A Twitter account with the handle @LizardSquad claimed responsibility for Sunday's attack and said it was meant to pressure Sony to invest more in the network.
"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," one post said.
Sony's was the victim of a notorious 2011 breach that compromised data of tens of millions of PlayStation Network users, dealing a major blow to plans for a looser network designed to allow for the connection of a range of Sony devices.
Since then it has invested heavily in securing the network, which it hopes can serve as a centerpiece of its plans to rebuild its business after years of losses in its flagship electronics operations.
Lizard Squad said it had also targeted servers of World of Warcraft video game maker Blizzard Entertainment, whose website was down. It threatened to attack Microsoft Corp's Xbox Live network, which some users said they had problems accessing on Sunday.
"We don't comment on the root cause of a specific issue, but as you can see on Xbox.com/status, the core Xbox LIVE services are up and running," Xbox spokesman David Dennis told Reuters.
Blizzard Entertainment was not immediately reachable for comment, though its customer support Twitter account said the company's servers were stabilizing.
Lizard Squad also tweeted to American Airlines on Sunday to say they had heard that explosives were on board a flight carrying Sony Online Entertainment President John Smedley.
That followed an earlier tweet from a game player's forum telling the airline: "I'm gonna send a bomb on your plane be ready for me tomorrow."
FBI spokesman Joshua Campbell said that the bureau was investigating the threat against American Airlines Flight 362, which was traveling from Dallas to San Diego. It was diverted to Phoenix Sky Harbor Airport, then continued on to San Diego, he said.
Daniel Clemens, founder of PacketNinjas, a company that helps businesses fight cyber attacks, said he expects the government will quickly catch the attackers.
Clemens, an expert in hacker activist denial-of-service attacks, said the Lizard Squad did not appear to be practicing good operational security, meaning it will be relatively easy for authorities to track them down.