© Reuters / Tyrone Siu
The US government has been spying on Huawei using the Foreign Intelligence Surveillance Act, prosecutors in the case against the firm's CFO Meng Wanzhou revealed, collecting "extremely voluminous" evidence via the secret channel.

While the substance of the evidence - described as "electronic surveillance and physical search pursuant to the Foreign Intelligence Surveillance Act" - was not disclosed, Assistant US Attorney Alex Solomon said it would require "classified handling" and that the government would require a protective order over whatever it shares with the defense.

FISA warrants can only be obtained in cases where the target is suspected of acting as an agent of a foreign power, meaning US authorities were already treating Huawei as an extension of the Chinese government in order to collect evidence Huawei was acting as an extension of the Chinese government.

Meng was arrested in Vancouver en route to Mexico and has been on house arrest pending extradition to the US, where she faces bank fraud and wire fraud charges stemming from Huawei's alleged violations of US sanctions against Iran and alleged theft of technology from US competitor T-Mobile. Meng and Huawei have pleaded not guilty to all charges, accusing the US of "political persecution" amid President Donald Trump's trade war with China.

Huawei filed its own lawsuit against the US government last month, alleging that banning federal agencies from purchasing its products is unconstitutional. The last National Defense Authorization Act specifically singled out Huawei and ZTE, another Chinese firm, to be excluded from federal contracts - a measure that constitutes unfair competition, Huawei claims.

The US has been on a mission to keep its allies - particularly Germany - from embracing Huawei technology, claiming the Chinese telecoms giant could have constructed undetectable backdoors in its equipment that would funnel data back to China. Their warnings have been largely ignored, possibly because the US itself was found to have been spying on German chancellor Angela Merkel and her ministers for over a decade, or because the US has failed to show evidence that Huawei is spying for the Chinese government. More recently, the US has threatened to withhold intel from allies who purchase Huawei tech to build their 5G networks.