Also that month, the Muhammad Islamic Center of Greater Hartford received a phone call from someone who threatened to burn it down, the authorities said. The person used racial slurs in the call, according to an imam at the center.

In April, three predominantly black churches were set on fire in Louisiana, and a gunman opened fire at a synagogue in Poway, Calif., during a service on the last day of Passover. The authorities were investigating whether the suspect in the shooting was the author of an anti-Semitic manifesto, in which the writer also claimed responsibility for the mosque fire in Escondido.

“Given recent attacks on houses of worship in this country and around the world, it is incumbent on law enforcement authorities to investigate the possibility of a bias motive for this arson,” Tark Aouadi, the executive director of the Council on American-Islamic Relations in Connecticut said in a statement about the New Haven fire.

Mr. Elevli said he had not heard of any threats targeting the mosque or its members before the fire. He said it had not crossed his mind that the mosque could have been targeted.

“This comes from nowhere,” he said. “It’s terrible for everybody.”

The mosque, at 531 Middletown Avenue, near Foxon Boulevard, has classrooms and apartments and opened in 2010, Mr. Elevli said. The mosque was under construction, and the building was largely empty when the fire occurred.

The mosque’s parent organization, the Diyanet Center of America, is a Maryland-based nonprofit that provides “religious, social and educational services to Turkish immigrants and Muslims,” according to the organization’s website.

Over the years, the New Haven mosque had become a place for families to gather, with educational programs for children on the weekends.