The Federal Security Service told AFP that it was unlikely to have been an act of terrorism.
Authorities said the incident took place in an outlet of the Maksidom chain of hardware and household goods stores.
Gas capsules with timers were found in three other Maksidom outlets in Russia's second city.
No explosives were found, and gas was only released in one of the targeted stores.
"The security services are inclined to believe this to be an act of hooliganism because so far there is no information that this could be a terrorist act," the FSB spokesman said.
Local police said it was most likely a related to a business dispute, according to a report by Russian news agency Interfax.
Officials said 78 people needed medical care and 66 were hospitalised after being exposed to the gas, identified as mercaptan, a compound of carbon, hydrogen and sulphur which is used to give odour to natural gas supplies.
Mercaptan can also be an ingredient in self-defence weapons, said police.
In some cases it can cause allergic reactions and is poisonous in large quantities.
Only around 15 people remain in hospital and no serious cases have been reported.
The gas escaped when the capsule was accidentally knocked over, the FSB said, while in the other two stores, on the outskirts of the city, the devices were discovered and destroyed.
All four shops were evacuated and closed as police investigate.
Russia, which has been fighting a brutal guerrilla war in the Caucasus region of Chechnya for most of the last decade, has suffered numerous large-scale terrorist attacks in the last few years, including the Beslan school siege and the downing of two airliners.
However, bloody business and criminal turf wars are also frequent.
On July, 21 people died in a fierce blaze at a shopping centre in the northwest city of Ukhta that police said was probably the result of business-related arson.
Bombings and dozens of shootings -- often unsolved -- have been also been blamed over recent years on mafia battles.