Kim Dotcom has asked a judge to order the United States to come clean on spying after a phone call glitch prompted fears he was victim to a surveillance system used by the FBI.
The Stingray could have spied on Dotcom, the Auckland District Court heard yesterday.
The system works as a fake cellphone tower, searching out a specific signal then hijacking call data connecting to or leaving the target phone.
Judge Nevin Dawson was told the use of Stingray would explain a bizarre call phenomenon that occurred during Dotcom's discussions with lawyer William Akel.
Dotcom heard his own voice playing back over his cellphone while talking to Mr Akel, the court heard.
In an affidavit to the court, Dotcom said he was talking legal strategy on August 2 and had just finished speaking. Expecting Mr Akel to respond, Dotcom said his lawyer went silent, to be replaced by a 15-second echo of Dotcom's own voice.
"Since the raid on my home I have become concerned that my communications have been subject to interception by security agencies," he said. He specifically named NZ and the US as among those he suspected.
An expert witness, former New Zealand police crime lab boss John Thackray, said the circumstance described by Dotcom could have resulted from surveillance.
Christine Gordon, QC, for the US, said assurances from US Attorney Jay Prabhu should meet any concerns over US conduct.
Dotcom and three others face extradition to the US on charges of criminal copyright violation relating to the Megaupload website.
Yesterday's hearing is part of the extradition proceedings and was brought by Dotcom. Judge Dawson reserved his decision.
What does Stingray do? - It pretends to be a cellphone tower, tricking cellphones into connecting to it. It is portable, the size of a small box and leaves no trace.
Who uses it? - The FBI is a big user of Stingray and can do so without a warrant.