"Russia, Russia, Russia is the chant when anything happens," Trump tweeted after Secretary of State Mike Pompeo said Russia is "pretty clearly" behind the attack.
Experts fear it will take months to kick elite hackers out of the U.S. government networks they have been quietly rifling through since as far back as March.
Officials did not say which agencies or infrastructure had been breached or what information taken in a March attack.
It only took the hacker five tries to guess the correct password, and he was kind enough to tell Donald Trump's team to activate two-factor authentication afterward.
The hack is so serious it led to a National Security Council meeting at the White House on Saturday, said one of the people familiar with the matter.
The social media platform said it would label, instead of block, tweets containing potentially hacked materials unless they're directly shared by hackers.
Officials say Graham Clark, 17, masterminded the hack of accounts belonging to people like Barack Obama, Elon Musk, Joe Biden and Jeff Bezos.
The Twitter accounts of several prominent politicians, tech bigwigs and companies -- including Joe Biden, Barack Obama and Apple -- were hit in the attack.
Even if the platform deters another attack, the hack is a lightning rod for disinformation.
Barack Obama, Jeff Bezos, Kanye West, Michael Bloomberg, Apple and Uber were also targeted in the massive cryptocurrency scheme.