New Haven Fire Chief John Alston said Monday investigators had found evidence the Sunday afternoon blaze at the Diyanet Mosque was "intentionally set" and a criminal investigation is underway.
He didn't disclose the nature of the evidence.
Alston says the one person inside when flames broke out escaped, and no one was hurt.
The fire occurred during the holy month of Ramadan. The front of the mosque was under construction, but some areas were still being used for worship.
Mosque president Haydar Elevli says area churches had offered his congregation a place to hold services.
Democratic Gov. Ned Lamont visited the mosque and said he finds "an attack like this especially hurtful and hateful."