Writing via the PS Blog, SCEA comms head Patrick Seybold said that the service was taken offline because of an "external intrusion".
"An external intrusion on our system has affected our PlayStation Network and Qriocity services," he wrote.
"In order to conduct a thorough investigation and to verify the smooth and secure operation of our network services going forward, we turned off PlayStation Network & Qriocity services on the evening of Wednesday, April 20."
He added: "Providing quality entertainment services to our customers and partners is our utmost priority. We are doing all we can to resolve this situation quickly, and we once again thank you for your patience. We will continue to update you promptly as we have additional information to share."
Both the US and European versions of PSN went down on Wednesday. Sony has since conceded that the disruption could last for days. The service remains unavailable in Europe.
According to previous figures, there are more than 75 million registered PSN accounts in 59 countries worldwide.