US President Donald Trump's Twitter account briefly vanished on Thursday but has since been restored, the social media company said.
An employee deactivated the @realdonaldtrump account, it said, clarifying that it had been their last day in the job.
The account was down for 11 minutes and Twitter is now investigating.
Tweets from Mr Trump, who has 41.7 million followers, have frequently caused controversy.
The latest incident has sparked debate about the security of the president's account, given the potential consequences of posts falsely attributed to Mr Trump being published.
However, @POTUS, the official account of the US president, was unaffected.
On Thursday evening, visitors to Mr Trump's page for a short time could only see a message that read "Sorry, that page doesn't exist!"
After the account was restored, Mr Trump's first tweet was about the Republican Party's tax cuts plan.
Twitter said it was investigating the problem and taking steps to avoid it happening again.
It later said: "Through our investigation we have learned that this was done by a Twitter customer support employee who did this on the employee's last day. We are conducting a full internal review."
'Bing, bing, bing'
Mr Trump joined Twitter in March 2009 and he has tweeted more than 36,000 times.
He has been actively using the social media platform to promote his policies and also attack his political opponents both during the presidential campaign in 2016 and since taking office in January.
In one interview he said that when someone said something about him, he was able to go "bing, bing, bing on Twitter" - and take care of it.
After he appeared to directly threaten North Korea with destruction in a tweet in September, Twitter was forced to justify allowing the post to stand.
It said that Mr Trump's tweet was "newsworthy".
In some of his other most controversial tweets:
He erroneously linked a rise in the UK crime rate to "radical Islamic terror"
He taunted FBI chief James Comey days before sacking him in May
End of Twitter post 3 by @realDonaldTrump
He admitted he had no such tapes of Mr Comey in June
Mr Trump's allies have also got into hot water over their use of Twitter.
Roger Stone, who advised him during his election campaign, was suspended from the network after he used abusive and homophobic language to target journalists, including a gay CNN presenter, Don Lemon.
He said he had been told by Twitter that he had violated its rules.
Mr Stone said he would sue Twitter for blocking his account.