Arid-ul-Islam was arrested during a raid at a guest house yesterday, Superintendent of Police Abdul Hameed said.
An automatic weapon was recovered from the possession of Ismal, who used a fake identity to stay at the guest house.
Islam was wanted in connection with the firing outside the Loreto College in Kolkata on February 15.
Hameed said that Islam not only admitted to the firing, but also stated that he belonged to Mirpur in Bangladesh.