The Muslim American Society of Minnesota says that someone threw an explosive device that started a fire in the imam's office at a mosque near Minneapolis.

The society said in a statement that attendees extinguished the fire at Dar Al-Farooq Islamic Center in Bloomington. Police say the explosion was reported at about 5 a.m. Saturday and there were no injuries.

There were 15 to 20 people were gathered for morning prayers when the explosion occurred. Bloomington police Chief Jeff Potts says that investigators are trying to determine the cause of the blast.

Asad Zaman, director of the society, told WCCO-TV that a witness saw someone standing by the imam's office window before the blast and also saw a truck flee the scene. He says the imam's window was broken and a small fire bomb was thrown inside.

The Star Tribune reports that Mohamed Omar, executive director of the Dar Al-Farooq Islamic Center in Bloomington, says the center occasionally receives threatening calls and emails.

Zaman says the organization is offering a $10,000 reward for information leading to an arrest and conviction.