Four men were arrested on suspicion of plotting to bomb a Moscow-to-St. Petersburg express train, Russian officials said. The suspects were detained last month, RIA Novosti reported Monday. The Federal Security Service accused the men of planning to bomb the express train Sapsan as it traveled by Khimki, a town near Moscow. The four men had been under the intelligence agency's surveillance. Agency Director Alexander Bortnikov told Russian President Dmitry Medvedev last month a "major act of terrorism" had been averted but didn't provide details at the time. Two years ago, a bomb explosion derailed a luxury Moscow-to-St. Petersburg train, killing 26 people.