BLOOMINGTON, Minn. (AP) — Someone apparently threw a bomb through the window of a suburban Minneapolis mosque on Saturday as people were preparing for morning prayers, damaging the imam's office but not injuring anyone, an official said.

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 blast happened at around 5 a.m. at the Dar Al-Farooq Islamic Center in Bloomington, south of Minneapolis. 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 in a news release.

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 reports that he said it may have been a fire bomb. Mohamed Omar, the center's executive director, said a worshipper saw a pickup truck speeding away afterward.

The mosque, which primarily serves people from the area's large Somali community, occasionally receives threatening calls and emails, Omar told the Star Tribune .

"It was 5 a.m.," Omar said. "The whole neighborhood 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, saying that he felt it on his "insides."

Yasir Abdalrahman, a worshipper at the mosque, said the explosion was "unimaginable."

"We came to this country for the same reason everyone else came here: freedom to worship," Abdalrahman said. "And that freedom is under threat. Every other American should be insulted by this."

An FBI spokesman didn't immediately reply to a phone message seeking further information about the investigation.

The mosque serves as a religious center and community organizing platform for Muslim activists and leaders in the area, said the society, which is offering a $10,000 reward for information that leads to an arrest or conviction.