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.
ISIS claimed responsibility for the blast, saying it was carried out by an ISIS "security detachment." But so far, ISIS has not given any evidence supporting its claim.
St. Petersburg, Russia's second-largest city, has been a target for at least one other explosion this year. Fourteen people were killed and dozens more were injured on April 3 when a suicide bomber detonated explosives in the city's metro system.
The April explosion, which has been linked to terrorism, occurred on a passenger train traveling through a tunnel in the city's center.
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