An explosion at a suburban Minnesota mosque was caused by a bomb thrown through a window as people were preparing for morning prayers, an official said.
No-one was hurt in the incident in Bloomington, south of Minneapolis, which damaged the imam's office.
The local police department tweeted that a preliminary investigation shows that a destructive device caused the explosion, "in violation of federal law", and that the FBI has taken the lead in the investigation.
The early-morning blast happened at the Dar Al-Farooq Islamic Centre in Bloomington.
There were 15 to 20 people inside the mosque when the blast happened, and worshippers managed to extinguish the blaze before firefighters arrived, the Muslim American Society of Minnesota said.
The society's director, Asad Zaman, 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 reported that he said it may have been a firebomb.
Mohamed Omar, the centre's executive director, said a worshipper saw a pick-up truck speeding away afterwards.
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 .
"It was 5am," Mr Omar said.
"The whole neighbourhood was calm. People were supposed to be sleeping, that's how peaceful this should be. I was shocked to learn this happened."
Trevin Miller, who lives across the street, told the Star Tribune that the explosion woke him up.
Yasir Abdalrahman, a worshipper at the mosque, said after the blast: "We came to this country for the same reason everyone else came here: freedom to worship.
"And that freedom is under threat. Every other American should be insulted by this."
The mosque serves as a religious centre and community organizing platform for Muslim activists and leaders in the area, said the society, which is offering a 10,000 dollar (£7,600) reward for information that leads to an arrest or conviction.