Investigators examined the site of an explosion at the Dar Al Farooq Islamic Center in Bloomington, Minn., on Saturday. David Joles/Star Tribune via Associated Press BLOOMINGTON, Minn. (AP) — The FBI was searching for suspects Saturday after an explosive device detonated at a suburban Minneapolis mosque as people were preparing for morning prayers, damaging a room but not causing any injuries, authorities and witnesses said.

The blast happened at around 5 a.m. at the Dar Al-Farooq Islamic Center in Bloomington, according to Bloomington Police Chief Jeff Potts. Windows of the imam's office were shattered, either by the blast or by an object thrown through them, the Minneapolis Star Tribune reported .

One worshipper saw a pickup truck speeding away shortly after the explosion, said Mohamed Omar, the center's executive director. He said the mosque, which primarily serves people from the area's large Somali community, occasionally receives threatening calls and emails.

"We came to this country for the same reason everyone else came here: freedom to worship," Yasir Abdalrahman, a worshipper at the mosque, told the newspaper. "And that freedom is under threat. Every other American should be insulted by this."

Asad Zaman, director of the Muslim American Society of Minnesota, described the attack as a firebombing.

Richard Thornton, special agent in charge of the FBI's Minneapolis Division, said during an afternoon news conference that the investigation will determine whether the incident was a hate crime and who may have been behind it.

Thornton said the explosion was caused by an "improvised explosive device," and that investigators have recovered components of the device to figure out how it was put together. But he didn't take questions during the news conference and declined to provide details about the device, citing the ongoing investigation, which is being led by the FBI.

Saturday's bombing comes amid a rise in reports of anti-Muslim incidents in the U.S., including arson attacks and vandalism at mosques, harassment of women wearing Muslim head coverings and bullying of Muslim schoolchildren. Just recently in Minnesota, an Islamic cemetery in Castle Rock Township reported it had been vandalized with spray painted profanities and swastikas.

The mosque in Bloomington, just south of Minneapolis, serves as a religious center and community organizing platform for Muslim activists and leaders in the area, according to the society. The group is offering a $10,000 reward for information that leads to an arrest or conviction.

A $10,000 reward also is being offered by the Minnesota chapter of the Council on American-Islamic Relations, or CAIR. The group said its national office is urging Islamic centers and mosques to step up security.

"If a bias motive is proven, this attack would represent another in a long list of hate incidents targeting Islamic institutions nationwide in recent months," said Amir Malik, the local chapter's civil rights director.

Along with a mosque, the building houses a community center that hosts computer classes, a basketball league, religious classes, lectures and other events.

Minnesota is home to the largest Somali community in the U.S., roughly 57,000 people, according to the latest census figures. The immigrants have been coming to Minnesota from their war-torn homeland since the 1990s, drawn by generous social services and the sense of community among the diaspora.