A forensic team on Monday collected samples from the site of a school building where a crude bomb exploded in West Bengal's Titagarh, officials said.

"We have collected some samples and those would be sent to the lab," an official of the team told reporters.

Four persons were arrested in connection with the blast on Sunday.

The crude bomb exploded on the roof of the school building on Saturday when classes were in progress.

There was no casualty in the incident, as students and teachers were in rooms located on the first two floors of the three-storeyed building, police had said.

The BJP has demanded a National Investigation Agency (NIA) probe into the explosion.