At the time, prosecutors allege, Mrs Heidrun was sitting in her study in front of a wireless transmitter that was receiving encoded messages on a shortwave frequency and was hooked up by cable to a computer, reports the German news website Spiegel Online.

BBC: Russia 'spies' go on trial in Germany

Clandestine agents in the United States monitoring the frequency on shortwave radio could decode the messages using a decryption program provided by the CuIS. Such methods were employed by defendants previously convicted of espionage on behalf of Cuba.

CENTERFORSECURITYPOLICY: Cuban Intelligence Operations in the US