At least 78 people were sickened in a poison gas attack in a store, possibly triggered by a business dispute, in the northern city of St. Petersburg on Monday, emergency officials and prosecutors said.

A spokesman for the Emergency Situations Ministry told the Interfax news agency that 78 people, most of them shop employees of the Maxidom chain, were poisoned and 66 people are being treated in hospitals, 12 of them in serious conditions.

Maxidom is a chain of stores that sell home repair tools.

"Suspicious boxes with wire were found in another two Maxidom outlets in the Krasnoselsky and Primorsky districts. All the outlets are now being examined," a source in the ministry's St. Petersburg department said.

They were capsules fitted with timers that released unidentified substance with a sharp smell of garlic, the Federal Security Service's city branch said.

The St. Petersburg Prosecutor's Office told Interfax preliminary data showed the substance was mercaptan, a compound that has a strong unpleasant odor and is used to odorize natural gas. Prosecutors are looking into the attack as a case of hooliganism.

While security officials ruled out a terrorist attack, local police said a business dispute could be behind the attack.

"The main version is unfair competition. Maxidom management turned to the Main Interior Department because they were receiving letters threatening to disrupt trading on New Year's eve," police spokesman Vyacheslav Stepchenko said.

Editor: Yan