A gas attack at a home-supply store in Russia's second-largest city left more than 70 people ill Monday, and devices with wires, timers and ampules were found in the other three outlets of the same chain, authorities said.

Police and government officials said they believed a commercial dispute or blackmail attempt was behind the incidents at the Maksidom chain. The incident caused no serious injuries but raised fears of attacks during the holiday shopping season despite assurances that terrorism was not involved.

Seventy-eight people sought medical care and 66 of them were hospitalized, Emergency Situations Ministry spokesman Viktor Beltsov said. St. Petersburg governor Valentina Matviyenko later said that all who sought medical help had been sent home and were not suffering any ill effects.

Matviyenko echoed law enforcement officials who said the incidents were probably a case of hooliganism or an effort by a competitor to hurt the business. Maksidom officials said they had received threats recently that sales would be disrupted before New Year's, when Russians traditionally give gifts.

"The first reaction is that it is one of the competitors of this store chain," Matviyenko said in televised comments.

The gas was preliminarily determined to be methyl mercaptan, St. Petersburg police spokesman Vyacheslav Stepchenko said. Methyl mercaptan is a gas with an acrid smell that is both naturally occurring and manufactured for use in plastics and pesticides, the US Health and Human Services' Agency for Toxic Substances and Disease Registry says.

The agency's Web site said little is known about the gas's potential health effects. One person exposed to very high concentrations of the gas went into a coma and died, the Web site says. But prominent Russia chemist Lev Fyodorov, head of an environmental group called For Chemical Safety, said on NTV television that the gas rarely has long-lasting effects.

Stepchenko said that a custodian at one branch of Maksidom discovered a suspicious box before the store's opening time and inside it found ampules - or small glass vials - attached to wires and a timer. The woman inadvertently broke one of the ampules and noticed a repulsive smell, but apparently was not sickened, he said.

All those who sought medical care were from another branch of the chain, where gas was released after employees heard a noise and police and security officers called to the scene found a mechanism with a timer attached to ampules that had shattered, Stepchenko said.

He said customers were among those sickened, but the ITAR-Tass news agency quoted unnamed city police officials as saying that the store had not opened when the gas was released and that all those affected were employees or police.

NTV showed cots set up in what it said was the cafeteria of an emergency hospital because the influx of patients from the store caused overflow, but said that none of those affected agreed to talk to reporters.

The patients complained of nausea and vomiting - as well as chest pain and high blood pressure which he said were probably a result of nervousness, nurse Alexei Afanasyev said. They reported an odor like garlic, he said.

Boxes with glass containers attached to timers were found in the chain's two other stores by employees who carried them outside and covered them with buckets; police explosives experts defused them, Stepchenko said.

Officials of the St. Petersburg chain, which sells furnishings, home-repair material and other domestic articles, told police they had received letters in recent weeks threatening to disrupt the company's sales during the holiday gift-buying period, Stepchenko said.

"According to store management, this was the work of competitors," he said in televised comments.

The cutthroat capitalism that enveloped Russia following the 1991 Soviet collapse led to business-related violence that still continues, but most efforts to undermine competitors' sales appear to have taken the form of negative advertising or potentially damaging rumors.

Russian media said that a business dispute might have been behind an explosion and fire that killed 24 people at a store in the northern city of Ukhta in July.