SpaceX announced an agreement on Thursday to purchase a nationwide spectrum portfolio as it to pushes its Starlink service deeper into the U.S. telecommunications market. Shares of AT&T, Verizon and T-Mobile tumbled in extended trading.

The deal involves the acquisition of spectrum portfolio from Grain Management, which specializes in digital infrastructure, and is subject to approval by the Federal Communications Commission. SpaceX said in a statement that it's a "license portfolio of up to 14 megahertz of paired spectrum in the 800 MHz band."

"This prime low-band spectrum addresses one of the key remaining technical gaps that will pave the way for Starlink Mobile to become a major mobile carrier in the US," SpaceX said.

In a post on X, SpaceX CEO Elon Musk called it a "very big deal."

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