Complex sentence: The spies were planted in the United States by the Russian Foreign Intelligence Service (known by its Russian abbreviation, "SVR"), and were the target of a multi-year investigation by the U.S. Federal Bureau of Investigation.
Split into simpler sentences: The spies were planted in the United States by the Russian Foreign Intelligence Service (known by its Russian abbreviation, "SVR"). They were the target of a multi-year investigation by the U.S. Federal Bureau of Investigation.