MOSCOW (AP) – A man was arrested Saturday in connection with a St. Petersburg supermarket bombing that wounded 18 people, Russia’s main domestic security agency said.

Eight people remain hospitalized after a device exploded Wednesday in a storage area for customers’ bags. Investigators said the homemade device contained 200 grams (7 ounces) of explosives and was rigged with shrapnel to cause more damage.

The Federal Security Service, or FSB, did not identify the suspect or provide any details about his motive. The agency said in a statement carried by Russian news agencies that he organized and carried out the attack on his own.

The Islamic State group claimed responsibility for the blast, but a member of the security committee in the lower house of Russia’s parliament cast doubt on the claim, saying it might have been an attempt by IS to gain publicity.

“Now, when the IS defeat in Syria seriously dented its image, it will try to restore its authority and claim responsibility for the attacks by others,” Adalbi Shakhgoshev said, according to Russian news wires.

The FSB said that it has handed over the suspect to the Investigative Committee, the nation’s top investigative agency. The FSB normally deals with terror suspects itself, and the transfer could indicate that the man had other motives.

The Interfax news agency reported that the suspect was a 35-year old local resident who said he was a member of an occult movement and was taking drugs.

Russian law enforcement agencies stopped short of immediately describing the supermarket blast as a terror attack, but President Vladimir Putin called it one Thursday. He added that he ordered security agencies to kill terror suspects on the spot if they resist arrest.

Putin emphasized that Russia’s two-year military campaign in Syria helped avert more terror attacks by eliminating militants who threatened Russia.

“What would have happened if those hundreds, thousands … had come back to us, trained, armed and well-prepared?” he said.

The Russian leader previously said that over 4,000 citizens from Russia and some 5,000 people from other ex-Soviet nations have joined the Islamic State group in Syria and Iraq.

In April, a suicide bombing in St. Petersburg’s subway left 16 people dead and wounded more than 50. Russian authorities identified the bomber as a 22-year old Kyrgyz-born Russian national.

Earlier this month, Putin declared victory in Syria and ordered a partial withdrawal of Russian troops from the country. Russia waged an air campaign in Syria since September 2015, helping President Bashar Assad’s forces rout both the IS and the rebels opposing his regime and recapture wide swathes of land.

Also earlier this month, Putin telephoned U.S. President Donald Trump to thank him for what the Kremlin described as a CIA tip that prevented more bombings in St. Petersburg, Putin’s hometown.

The Federal Security Service said seven IS-linked suspects were arrested in connection to the alleged plot. The Kremlin said the suspects had planned to bomb the landmark Kazan Cathedral and other crowded sites.