Several people were arrested on Thursday on suspicion of being accomplices of the man behind Monday’s St. Petersburg metro bombing, news agency Interfax reported on Thursday, citing a law enforcement source.

It said the detainees’ links with the suicide bomber were being verified.

Russian investigators said they had searched a flat of acquaintances of the suspected metro bomber, as they probe the attack that left 13 people dead.

“Objects relevant to the investigation were found during the search of the apartment where these people lived,” the Investigative Committee said. “They were all confiscated and sent for analysis.”

Russia’s Federal Security Service (FSB) said, according to Russian news agencies, that the object found early Thursday in an apartment building on St. Petersburg’s eastern outskirts could contain explosives. Residents have been evacuated and explosives experts have started working on the site.

Police in the city are on high alert following Monday’s explosion that killed the attacker and 13 other people and wounded some 55.

Police on Wednesday arrested eight Central Asian migrants suspected of acting as recruiters for the ISIS group and al-Qaeda’s Syria branch. The investigators found no immediate evidence of their involvement in the subway attack.

Also on Thursday, a man was hurt in an explosion in the southern Russian city of Rostov-on-Don, a law enforcement source told TASS news agency.

At around 6:30am (0330 GMT), the man discovered a bag containing a torch and when he tried to switch it on, it exploded and tore off his hand, the TASS official news agency quoted a local policeman as saying.

REN-TV cited witnesses as saying that the explosion happened near a school on Sadovaya Street.