Authorities said criminals were the likely perpetrators of the attack, which sickened some 78 people. Several of them were hospitalized, but none with life-threatening conditions.
Officials say those who fell ill were at a store of the Maxidom chain of household goods. Gas containers with timers were found in two other Maxidom shops in Russia's second largest city, but were rendered harmless.
St. Petersburg Governor Valentina Matviyenko said the incidents were probably a case of hooliganism or an attempt by a competitor to hurt Maksidom's business. The chain had reportedly received letters threatening to disrupt holiday sales. Officials were quoted as saying that the letters had accused the chain of "dishonest competition."
A spokesman for the Federal Security Service (FSB) said the gas appears to have been methyl mercaptan. That gas is highly irritating when it contacts moist tissues such as the eyes, skin, and the upper respiratory tract.
(compiled from agency reports)