Worshippers were inside the Dar Al-Farooq Islamic Center in Bloomington, Minnesota, when the explosion ripped through the imam’s office.
It is believed a window was broken and a explosive device hurled inside.
The FBI are investigating – but said it is “too early” to call it a hate crime.
Police and federal agents are trying to determine the exact cause of the blast – which is thought to be a bomb.
Mohamed Omar, executive director of the center, said the blast happened in the imam’s office during the day’s first prayer.
Asad Zaman, the executive director of Muslim American Society of Minnesota, offered a $10,000 reward for information on the incident.
The building sustained fire and smoke damage, but there were no injuries.
Zaman said a member of the congregation saw a pickup truck speeding away from the mosque after the explosion.
Local religious leaders are planning to meet to discuss the incident which happend at 5.05am UTC.
The United States Bureau of Alcohol, Tabacco, Firearms and Explosives are also investigating.
Saturday August 5, 2017