Twitter headquarters in San Francisco.

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Twitter Inc. shares retreated slightly in after-hours trading Thursday as the social-media platform experienced a massive and as-yet-unexplained outage.

Twitter’s feed went down around 5:30 p.m. Eastern, with users unable to post new tweets or see anything posted past that time.

Tens of thousands of users reported problems to

“We are currently investigating this issue,” Twitter said on its API website.

Around 6:17 p.m., Twitter said in a statement: “We know people are having trouble Tweeting and using Twitter. We’re working to fix this issue as quickly as possible. We’ll share more when we have it and Tweet from @TwitterSupport when we can – stay tuned.”

A company spokeswoman told MarketWatch, “We have no evidence of a security breach or hack, and we’re currently investigating internal causes. We’re working to fix this issue as quickly as possible.”

The outage is the first major glitch to strike the platform since July, when hackers took over the accounts of a number of high-profile users. It also comes a day after Twitter became embroiled in controversy for blocking links to a questionable New York Post article about Hunter Biden, citing its hacked materials policy.

Twitter shares

are up 44% year to date, compared to the S&P 500’s

nearly 8% gain this year.

MarketWatch’s Jon Swartz contributed to this report.

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