US authorities decided to end a decade of investigation and arrest suspects in an 11-person alleged Russian spy ring because of a threat to the case, a report said yesterday.

"Something happened that was going to affect them all," a senior law enforcement official told The Washington Post, without disclosing what occurred to prompt the arrests last Sunday, which the official said were required to "protect the cases".

The first probes into the alleged conspiracy to perpetrate 'deep cover' Russian espionage in the US came in 2000, according to a complaint filed against the suspects.

By 2006, searches of property had been carried out and US officials were monitoring encrypted messages between some of the 11 suspects and their Russian spymasters, including by placing recording devices in the homes of some of the accused spies, the Justice Department said in its legal complaint last Monday.

The official told the Post, however, that the arrests were put off because "there is always something else to be learned," with US authorities gathering counterintelligence on Moscow's espionage tactics and techniques.