Dozens of people have been treated and some of them taken to hospital after an unidentified gas was released in a shop in the Russian city of St Petersburg.
Canisters were found in four Maksidom shops
The emergency situations ministry said the incident happened in an outlet of the Maksidom chain.
Gas capsules with timers were found in three other Maksidom shops in the northern city, officials said.
Security officials said the incidents were most likely caused by criminals, rather than terrorists.
"The security services are inclined to believe this is an act of hooliganism because so far there is no information that this could be a terrorist act," a spokesman for the Federal Security Service said.
Criminal attacks on businesses are common in Russia. Twenty people died in an arson attack on a shop in the northern city of Ukhta in July.
No serious cases
A police spokesman told AP news agency that the Maksidom chain, which sells furnishings and hardware, had received letters threatening to disrupt sales during the holiday period.
Officials said 78 people sought medical care as a result of the gas, 66 of whom were taken to hospital. More than 50 were released soon afterwards.
No serious cases have been reported.
Victims of the attack reported an unusual, garlicky smell.
Preliminary reports said the substance used was methyl mercaptan, a compound of carbon, hydrogen and sulphur.
The substance is added to natural gas to give it its odour and is poisonous in large quantities.