An "improvised explosive device " caused blast at a suburban Minneapolis mosque when worshippers were gathering for morning prayers, according to the FBI.
No one has been arrested yet. Congregants and neighbours expressed relief that there were no injuries, but also reacted with shock and dismay.
According to authorities, no one was hurt in the explosion at the Bloomington Islamic Center, but heavily damaged an imam's officeand sent smoke wafting through the large building.
Windows in the office were shattered, either by the blast or by an object thrown through them.
When police arrived, they found smoke and fire damage to the building, said Bloomington Police Chief Jeff Potts.
Agents from the FBI and the Federal Bureau of Alcohol, Tobacco, Firearms and Explosives soon joined the investigation.
At a news conference Special Agent in Charge Richard Thornton said an "improvised explosive device" caused the blast, but that investigators still must determine "who and why."
Mohamed Omar, the centre's executive director, who was in the building when the explosion erupted, said one worshipper saw a pickup truck speed out of the parking lot after the blast.
At a news conference that included clergy from several faiths, Asad Zaman, director of the local Muslim American Society, described the attack as a firebombing.
"Something blew up, we don't know what, but there was a lot of smoke, and the fire suppression system kicked in and stopped the fire," he said.
Neighbours reported waking up to a loud bang or pop, some even feeling the concussion reverberate through windows.
Minnesota law enforcement reported 14 anti-Muslim bias incidents last year, according to the Bureau of Criminal Apprehension, a record high even as other categories have been on the decline.
Governr Mark Dayton decried the incident and vowed to stay in close tough with law enforcement about the investigation.
"Every place of worship, for all Minnesotans of every faith and culture, must be sacred and safe," he said.
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