WASHINGTON, Feb 3 (Reuters) - Democratic Senator Maria Cantwell on Tuesday said Verizon and AT&T are blocking release of key documents about an alleged massive Chinese spying operation that infiltrated U.S. telecommunications networks known as Salt Typhoon and wants their CEOs to appear before Congress to answer questions.

Cantwell asked both companies to turn over security assessments conducted by Alphabet cybersecurity unit Mandiant. She said Mandiant refused to provide the requested network security assessments, apparently at the direction of AT&T and Verizon.

“If AT&T and Verizon are not going to provide Congress key documentation voluntarily, then I believe this committee must promptly convene a hearing with their CEOs so they can explain why Americans should have confidence in the security of their networks amid mounting evidence that the Salt Typhoon hackers remain active and undeterred,” Cantwell said in a letter to Senate Commerce Chair Ted Cruz.

(Reporting by David Shepardson in Washington)

((Email David.Shepardson@thomsonreuters.com 202-579-6093))