BLOOMINGTON, Minn. - Someone apparently threw a bomb through the window of a suburban Minneapolis mosque on Saturday as people were preparing for morning prayers, damaging the imam's office but not injuring anyone, an official said.

The blast happened at around 5 a.m. at the Dar Al-Farooq Islamic Center in Bloomington. Police Chief Jeff Potts said at a news conference that investigators were trying to determine the cause of the blast.

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Asad Zaman, director of the Muslim American Society of Minnesota, said at a news conference that a witness saw something being thrown at the imam's office window from a "van or truck" before the blast. WCCO-TV reports that he said it may have been a fire bomb. Mohamed Omar, the center's executive director, said a worshipper saw a pickup truck speeding away afterward.

There were 15 to 20 people inside the mosque when the blast happened. Worshippers managed to extinguish the blaze before firefighters arrived, the society said in a news release.

The mosque, which primarily serves people from the area's large Somali community, occasionally receives threatening calls and emails, its executive director, Mohamed Omar, told the Star Tribune .

The mosque serves as a religious center and community organizing platform for Muslim activists and leaders in the area, said the society, which is offering a $10,000 reward for information that leads to an arrest or conviction.

The FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives are assisting in the investigation.