Tesla Inc. Chief Executive Officer Elon Musk said it’s “time to break up Amazon” in a tweet Thursday, escalating a rivalry with Amazon.com Inc. CEO Jeff Bezos, another billionaire investing in space exploration.

“Monopolies are wrong,” Musk tweeted while tagging Bezos, the world’s wealthiest man. The online retailer is among tech companies being scrutinized by federal regulators and lawmakers for the increasing size and the scope of its business

Musk’s post came in response to a tweet from a writer who said his book titled “Unreported Truths About COVID-19 and The Lockdown” was being removed from Amazon’s Kindle publishing division for violating unspecified guidelines.

An Amazon spokeswoman said the book was removed in error and is being reinstated. “We have notified the author,” she said in an email.

Last year, a Space Exploration Technologies Corp. executive said Amazon’s effort to build a constellation of broadband internet satellites was years behind the closely held company. Musk founded SpaceX two years after Bezos started rival manufacturer Blue Origin.

With more than 35 million followers, Musk is a prolific tweeter. He has been criticized in the past for his posts on various subjects ranging from the coronavirus outbreak to Tesla’s stock price.

CORRECTION: A previous version of this Bloomberg News story misstated the timing of SpaceX and Blue Origin's founding.