A mosque in the state of Minnesota was bombed early Saturday, while Muslims inside gathered to perform morning prayer.

Axar.az reports citing AA.

Bloomington police said the bomb damaged only the imam's office at the Dar Al Farooq Islamic Center and worshippers extinguished the blaze before firefighters arrived. The FBI has is investigating.

"A witness saw something being thrown at the imam's office window from a van or truck before the blast," Asad Zaman, Director of the Muslim American Society of Minnesota, said at a news conference. The mosque’s executive director Mohamed Omar added that the vehicle immediately sped away.

The predominant Somali mosque, like many other mosques around the country, has received threatening calls and emails, Omar told the Star Tribune.

"It was 5 a.m. (0900GMT)," he said. "The whole neighborhood was calm. People were supposed to be sleeping, that's how peaceful this should be. I was shocked to learn this happened."

The Council on American Islamic Relations (CAIR), announced a $10,000 reward for information leading to the arrest and conviction of the attacker.

It’s national office also urged mosques and Islamic centers nationwide to increase security.

"The incident is a symptom of the growing atmosphere of Islamophobia in the U.S," the group’s communication director, Ibrahim Hooper, told Anadolu Agency. "Because whenever you look at what would be a possible motive in this kind of case, it seems that Islamophobia would be a main type of motivation that you would look at."

Hooper said rewards have been successful in the past to receive information leading to an arrest.

"Because often they brag to people and friends, their relatives, that they've done this and they are proud of it. Then somebody eventually, because of the incentive of the reward, offers information to the police and they are arrested," he added.