Story highlights 18 people were injured in the explosion at a St. Petersburg supermarket

ISIS claimed responsibility for the attack, but provided no evidence

Moscow (CNN) Russian authorities say they have detained a suspect believed to be responsible for this week's supermarket blast in St. Petersburg.

The suspect, whose identity has not been released, was detained Saturday, Russia's Federal Security Service (FSB) said in a statement. The suspect was under interrogation, the Russian state news agency TASS reported.

The explosion took place Wednesday at a Perekrestok supermarket in St. Petersburg. At least 18 people were injured, state-run media reported.

Preliminary information suggests an explosive device containing 200 grams (7 ounces) of TNT was inside a locker at the supermarket, state-run media said.

Shortly after the explosion, Russian President Vladimir Putin called the incident a terror attack.

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