More than 70 people were sickened after gas was released today in a St Petersburg chain store and boxes with glass containers attached to wires were found in three other outlets of the same store.
Police said they believed a commercial dispute or blackmail attempt was behind the incidents, and Russian news agencies quoted the city's top official as saying that authorities had ruled out terrorism.
Seventy-eight people sought medical care and 66 of them were taken to hospital, Emergency Situations Ministry spokesman Viktor Beltsov said.
City governor Valentina Matviyenko later said that all who sought medical help were sent home and were not suffering any ill effects.
Mr Matviyenko said the incidents were not terrorism and that law enforcement authorities believed they were either a case of simple hooliganism or an effort to compromise a competitor, the Interfax and RIA-Novosti new agencies reported.
The gas was preliminarily determined to be methyl mercaptan, St Petersburg police spokesman Vyacheslav Stepchenko said.
Methyl mercaptan is a gas smelling like rotten cabbage 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.