The newspaper quoted an unidentified official as saying a 23-year-old Russian man had been detained, but that there was insufficient evidence to charge him with a crime. He was being held at an undisclosed location pending deportation.
FBI agents reportedly became suspicious of the man and started monitoring his activities -- which were not revealed in the article --upon his arrival in the U.S. in October of 2009.
The 12th suspect was living in the U.S. on a valid visa, according to the Journal, which was revoked one day before 10 suspects in the alleged spy-ring were rounded up by agents in and around New York.
His name reportedly surfaced as part of the years-long investigation into the espionage, but FBI agents had their doubts as to his level of complicity.
The 10 suspects arrested and charged in U.S. courts with working for Russia's intelligence agency were flown out of the country last week as part of a spy-swap for four people who had been locked up on various charges in Russia.
The 11th suspect was arrested in the Mediterranean island nation of Cyprus, but disappeared after posting a $32,500 bail in cash.
More on the spy case: