WASHINGTON (AP) — National Intelligence Director James Clapper says it appears that a "non-state actor" was behind a massive cyberattack last week that briefly blocked access to websites including Twitter and Netflix.

Clapper says investigators are gathering a lot of data and preliminary indications are that a non-state actor is to blame. But he says he wouldn't want to completely rule out whether a nation state might have been behind it or not.

Clapper spoke Tuesday at the Council on Foreign Relations in New York.

Last Friday, cyberattacks crippled a major internet firm, repeatedly disrupting the availability of popular websites across the United States. Members of a shadowy hacker group that calls itself New World Hackers claimed responsibility for the attack, but that claim could not be verified.