Former culture minister Michael McGimpsey said he believed the spyring had "penetrated to the very heart of the security situation in the Northern Ireland Office".
Aged 54 or 55, he is depicted by US prosecutors as the alleged spy ring's shadowy money man, visiting the US as early as 2004 to deliver bags of cash to its members.
Yemen said in July it had arrested members of a spyring led by a former commander in Iran's Revolutionary Guard, the state news agency Saba said, adding that the cell had operated in the Horn of Africa as well as Yemen.
The BBC's Stephen Evans in Berlin that there is speculation about whether the authorities were alerted to the German couple by the same person - whoever that may be - who told the FBI in the United States about the Russian spyring uncovered there involving Anna Chapman.