No one was injured in the explosion, which caused a fire in the imam's office
Bomb suspected in morning blast at US mosque
A mosque in suburban Minneapolis, Minnesota was targeted by an apparent bomb attack on Saturday as worshippers were preparing for morning prayers.
The blast at about 5am caused a fire the imam's office but did not injure anyone, community officials said.
Police chief Jeff Potts said investigators were trying to determine the cause of the blast at the Dar Al Farooq Islamic Centre in Bloomington.
Asad Zaman, director of the Muslim American Society of Minnesota, said a witness saw something being thrown at the imam's office window from a "van or truck" before the blast that may have been a fire bomb.
Mohamed Omar, the centre'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.
The mosque, which primarily serves people from the area's large Somali community, occasionally receives threatening calls and emails, Mr Omar told the Star Tribune newspaper.
The mosque serves as a religious centre and community organising platform for Muslim activists and leaders in the area, the society said.
It is offering a US$10,000 (Dh36,700) 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.