Swedish police said on Saturday that they had arrested a 39-year-old Uzbekistan-born man they believed had hijacked a beer truck and carried out a terrorist attack by driving the truck into a crowd of people in Stockholm the day before, killing four and injuring at least 15 others.

Prosecutors and police officials did not identify the suspect, but Anders Thornberg, head of the Swedish Security Service, said at a news conference that the man had been on authorities' radar some time ago.

Thornberg said that the agency had looked into information it received on the suspect last year, but that it had not led to anything. He said the suspect was not on any current list of people being monitored.

The Swedish national broadcaster SVT reported that a bag with explosives had been found in the truck used in the attack.