In a statement on August 31, the FSB said that a powerful homemade bomb was found in the home of one of the suspects and claimed that he acknowledged he was planning a suicide attack.
It said the other suspect had posted a video on the Internet in which he swore allegiance to the extremist group Islamic State (IS) and said he was planning to attack civilians with a bladed weapon.
An FSB video carried by state-run Russian media showed a man saying that he planned to "commit jihad" and carry out a bomb attack "on the orders of Islamic State." It also showed an ax it said the other suspect was planning to use.
There was no way to independently verify the accuracy of the FSB statement or the authenticity of the video. Russian authorities frequently report that they have foiled terror plots ahead of major public events or holidays.
Comment: If only RFE/RL were so skeptical when it came to terrorism cases in the U.S.: "There was no way to independently verify the account of the FBI statement or the authenticity of their evidence. American authorities frequently report that they have foiled terror plots which they have in fact created."
The first day of school coincides with the Muslim holiday of Eid al-Adha, widely known in Russia and the mostly Muslim former Soviet republics of Central Asia as Kurban Bairam.
A hostage-taking attack on a school in the southern Russian town of Beslan on September 1, 2004, ended two days later with a botched rescue bid and led to the deaths of 334 people, more than half of them children.
On April 3 of this year, a blast that authorities said was a suicide bombing carried out by an ethnic Uzbek Russian citizen born in Kyrgyzstan killed 16 people on a subway car in St. Petersburg.
President Vladimir Putin said in April that some 9,000 militants, about half of them from Russia and half from Central Asia, have joined IS in Syria.
Putin has said that a crucial goal of Russia's military campaign in Syria is to kill militants there and prevent them from returning home.
In the months after Russia began a campaign of air strikes in Syria in September 2015, Western officials said it mainly targeted not IS militants but other opponents of President Bashar al-Assad.