Moscow ? Scores of people were sickened Monday morning after a pungent substance used to odorize natural gas was deliberately released in a St. Petersburg home repair store, police said.

Canisters attached to wires and timers were later found at three other branches of the same chain of Maxidom stores in the city, officials said.

Police linked the attack to business rivalries.

“The main reason for the poisoning in Maxidom stores in St. Petersburg is dishonest competition,” a spokesman for the St. Petersburg police told the Russian news agency Interfax. “Management of Maxidom contacted the Interior Ministry not long ago complaining that they had started to receive letters with threats to spoil their pre-New Year trade.”

Officials said 78 shoppers and store employees needed medical attention, but no one was seriously injured. Viktor Beltsov, a spokesman for the Emergency Situations Ministry, said 66 of the 78 people who sought medical care were admitted to hospitals. All were released by Monday evening, officials said.

Police identified the released substance as mecaptan, a compound of carbon, hydrogen and sulfur, which is used to give a smell to odorless natural gas to help consumers detect leaks.