SpaceX’s Starlink suffered one of its biggest international outages on Thursday when an internal software failure knocked tens of thousands of users offline, a rare disruption for Elon Musk’s powerful satellite internet system.

Users in the U.S. and Europe began experiencing the outage at around 3 p.m. EDT (1900 GMT), according to Downdetector, a crowdsourced outage tracker that said as many as 61,000 user reports to the site were made.

Starlink, which has more than 6 million users across roughly 140 countries and territories, later acknowledged the outage on its X account and said “we are actively implementing a solution.”

Starlink service mostly resumed after 2.5 hours, Michael Nicolls, Starlink vice president of Starlink Engineering, wrote on X.

“The outage was due to failure of key internal software services that operate the core network,” Nicolls said, apologizing for the disruption and vowing to find its root cause.

Musk had also apologized: “Sorry for the outage. SpaceX will remedy root cause to ensure it doesn’t happen again,” the SpaceX CEO wrote on X.

The outage was a rare hiccup for SpaceX’s most commercially sensitive business that had experts speculating whether the service, known for its resilience and rapid growth, was beset by a glitch, a botched software update or even a cyberattack.

Doug Madory, an expert at the internet analysis firm Kentik, said the outage was global and that such a sweeping interruption was unusual.

“This is likely the longest outage ever for Starlink, at least while it became a major service provider,” Madory said.

As Starlink gained more users, SpaceX has focused heavily in recent months on updating its network to accommodate demands for higher speed and bandwidth.

The company in a partnership with T-Mobile is also expanding the constellation with larger, more powerful satellites to offer direct-to-cell text messaging services, a line of business in which mobile phone users can send emergency text messages through the network in rural areas.

SpaceX has launched more than 8,000 Starlink satellites since 2020, building a uniquely distributed network in low-Earth orbit that has attracted intense demand from militaries, transportation industries and consumers in rural areas with poor access to traditional, fiber-based internet.

“I’d speculate this is a bad software update, not entirely dissimilar to the CrowdStrike mess with Windows last year, or a cyberattack,” said Gregory Falco, director of a space and cybersecurity laboratory at Cornell University.

An update to CrowdStrike’s widely used cybersecurity software led to worldwide flight cancellations and impacted industries around the globe in July last year. The outage disrupted internet services, affecting 8.5 million Microsoft Windows devices.

It was unclear whether Thursday’s outage affected SpaceX’s other satellite-based services that rely on the Starlink network. Starshield, the company’s military satellite business unit, has billions of dollars’ worth of contracts with the Pentagon and U.S. intelligence agencies.

Separately, Reuters reported on Friday that Musk ordered a partial shutdown of Starlink during a pivotal push by Ukraine to retake territory in its war with Russia in late September 2022.

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(Reporting by Juby Babu in Mexico City, and Raphael Satter and Joey Roulette in Washington; Editing by Anil D’Silva, Matthew Lewis and Toby Chopra)