STOCKHOLM — A bus exploded on Sunday morning in the heart of Stockholm after ramming into a barrier near the entrance of a tunnel in the Swedish capital.

No passengers were on the bus, the Swedish newspaper Aftonbladet reported, but the driver was taken to a hospital.

Emergency services received calls about the crash at Klara Tunnel in central Stockholm about 11:25 a.m. local time.

A witness, Björn Wallentin, told Aftonbladet: “I heard a big explosion and then several smaller ones. It sounded like the scaffolding of a building falling down. I thought it was a terror attack.”