The FBI is looking for suspects in an explosion at a Minnesota mosque where worshippers had assembled for morning prayers. Officials are investigating whether the explosion was a hate crime.

The blast went off early Saturday morning at the Dar Al-Farooq Islamic Center in the Minneapolis suburb of Bloomington, according to Bloomington Police Chief Jeff Potts. A room in the mosque was damaged, but no people were harmed.

Richard Thornton, special agent in charge of the FBI's Minneapolis Division, said the blast appeared to have been caused by an improvised explosive device. The FBI recovered pieces of the device from the scene.

The agency is now looking for the person responsible for the explosion, and seeking to determine whether it was a hate crime.

“At this point, our focus is to determine who and why,” Mr Thornton said at a press conference. “Is it a hate crime? Is it an act of terror?"

They FBI have so far interviewed witnesses, sent evidence to labs, and collected video and cellphone data for analysis.

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The Bloomington Police thanked the FBI and the community as a whole for their help, tweeting: "Thank you Bloomington community for your kind words & support today. We will always be here for you & will serve to the best of our ability."

Department of Homeland Security Secretary Elaine Duke is also aware of the situation, and is in contact with federal and local authorities.

“The Department of Homeland Security fully supports the rights of all to freely and safely worship the faith of their choosing and we vigorously condemn such attacks on any religious institution,” the DHS said in a statement. “We are thankful that there were no injuries, but that does not diminish the serious nature of this act.”

Neighbours in the sleepy suburb reported waking up to a loud bang on Saturday morning. Windows in the imam's office at the mosque were shattered, and smoke poured through the building. Asad Zaman, director of the local Muslim American Society, described the attack as a "firebombing".

Mohamed Omar, the centre's executive director, said one member saw a pickup truck speeding away from the scene shortly after the explosion. He added that the mosque has received threatening phone calls and emails in the past.

The number of anti-Muslim hate crimes has almost doubled this year compared to the same time period in 2016, according to the Council on American-Islamic Relations. The Council documented 35 attacks on mosques in the first three months of this year.

One regular worshipper at Dar Al-Farooq condemned Saturday's attacks in an interview with the Minneapolis Star Tribune.