The deadly string of apartment block bombings could have been worse. Amid this climate of crisis, on September 22 at 9:15 pm, an alert citizen of Ryazan coming home from work watched three people load several large bags of powder from a mysteriously marked car into the basement of his apartment building. He called it in to local police, and the Interior Ministry's UVD division in Ryazan confirmed the worst fears – the bags were filled with the explosive Hexogen, which had taken out the other buildings, and connected to a timer set to spark at 5:30 am. They evacuated the building and defused the bomb without incident, and the near-miss was all the buzz on the morning of the 23rd. This time the car was seen, its passengers identifiable, and the hunt for suspects was on. By the 24th they were reported as cornered, their car found and arrests imminent. 
Curiously, new FSB chief Patrushev revealed on the 24th that the bags had contained only sugar and had been planted as a readiness drill to see if people would catch the next attack in time. It was perhaps a ill-advised in its timing, but it was only a drill, and they found the sugar and succeded splendidly. Give yourselves a pat on the back. Of course since the ‘drill’ was an FSB operation, the Chechen "terrorists" who unloaded the bags were actually just FSB employees and so went unmolested as the government closed its case. FSB spokesman Alexander Zdanovich led the media campaign to explain away the incident as the official inestigation confirmed the official story and the official suspects were dragged before a kangaroo court and convicted.
FSB officers went to the building the "drill" was run on: one woman said "several people from the FSB came to see us, led by a colonel. They apologized. They said that they hadn't known anything either."  But the FSB later rebuffed lawsuits by people who had heart attacks and suffered long-term stress from the scare. Officially, the Ryazan incident, first framed as a failed attack, has been largely erased. For example, the Wikipedia article sites the duration of the September bombing spree as two weeks - Sept. 4-16. Ryazan is thus not part of it.
On the 23rd, as it still looked like terrorism and with the first air raids commencing in Chechnya, PM Putin stated about the failed "attack:” “As for the events in Ryazan, I don't think there was any kind of failure involved. […] This is absolutely the correct response. No panic, no sympathy for the bandits. This is the mood for fighting them to the very end. Until we win. And we shall win.”  He knew by then to start hinting at a readiness drill, but to still keep it vague enough to appear as maybe-terrorism to feed the all-important war drive, which would partly bury the following day's FSB "admission" long enough to let it get obscured beneath the official investigation.
 Reynolds, Maura. “Ryazan Fears Darker Truth of Bombings.” The Moscow Times. January 18, 2000.
 “The Ryazan Story.” Excerpts from "Blowing Up Russia: Terror from Within" by Yuri Felshtinsky and Alexander Litvinenko
 Borisova, Yevgenia. "No Proof Chechens Blew Up Buildings." Moscow Times. March 17, 2000. http://www.cdi.org/russia/johnson/4174.html
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