A blast caused by what the FBI called "an improvised explosive device" rocked a Bloomington Islamic center before dawn Saturday, just as a small group of Muslim worshipers had gathered for the day's first round of prayers.

No one was hurt in the explosion, which heavily damaged an imam's office at the Dar Al Farooq Center and sent smoke wafting through the large building. Windows in the office were shattered, either by the blast or by an object thrown through them.

The blast was reported at 5:05 a.m. as about a dozen people gathered in a room nearby for morning prayers and jolted awake many residents of the neighborhood. Congregants and neighbors expressed relief that there were no injuries, but also reacted with shock and dismay.

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. A large area outside the center was taped off as investigators, including members of the FBI's Joint Terrorism Task Force, combed through the grass.

At an early evening 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."

"The post-blast environment is very detailed," he said. "You search the wide area in an attempt to find as many components as you can of the device to help us understand how the device was made. That process is substantially complete. … It was an improvised explosive device that was set off early this morning."

Witnesses have been interviewed, evidence has been sent to various labs, and video and cellphone data are being analyzed, he said. He asked that anyone with information contact 1-800-CALL-FBI.

The U.S. Department of Homeland Security issued a news release saying that acting DHS Secretary Elaine Duke was aware of the explosion and was "in close contact with federal, state and local authorities and local community leaders as the investigation into this matter continues."

"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," DHS said. "We are thankful that there were no injuries, but that does not diminish the serious nature of this act."

Even before the FBI update, congregants said the blast appeared to be a hate crime. Mohamed Omar, the center's executive director, who was in the building when the explosion erupted, said one worshiper saw a pickup truck speed out of the parking lot after the blast.

At a noon 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.

Neighbors reported waking up to a loud bang or pop, some even feeling the concussion reverberate through windows.

Trevin Miller, who lives across the street, said he has heard occasional fireworks in the neighborhood, but nothing like Saturday's explosion.

"It woke us up instead of my alarm," he said. "It was loud; it was kind of like a firework-car crash-gunshot. It kind of shook me — like, you could feel it. I thought maybe somebody drove through our house or something. … I felt it on my insides."

Miller added, "I have a daughter that usually lives with me, and to wake up to all this, it's like, what the hell, this shouldn't be happening right at our doorstep."

Yasir Abdalrahman, who has been worshiping at the mosque for two years, said it serves Muslims from around the metro area, and that children often attend weekend "dugsi" services to learn to memorize and recite the Qur'an, he said. Fajr, the first prayer of the day, usually involves older worshipers who live near the center, he said.

"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."

Omar said the center occasionally receives threatening or hateful calls and e-mails. Usually, he said, callers say "that we shouldn't be here, that we are a burden to the community or we are harming it."

The building was once the site of Northgate Elementary School and Concordia High School. It also served as a worship space for Maranatha Community Church. The Dar Al Farooq Center bought it in 2011 to use as a place of worship, community center and day care site.

Some area residents opposed its establishment, and there have been occasional complaints about noise, traffic and parking problems. But Saturday's attack was the first such incident at the center.

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.

At the interfaith news conference, faith leaders spoke out against any attack on Minnesota Muslims.

"An attack on a mosque is an attack on a synagogue is an attack on a church is an attack on all faith communities," said the Rev. Curtiss DeYoung, CEO of the Minnesota Council of Churches. "So we stand with you."

Nausheena Hussain, executive director of Reviving the Islamic Sisterhood for Empowerment, called for more interfaith dialogues. "Have more hard conversations around what is happening around us today," she said. "We need your help now more than ever to continue to make Minnesota a welcoming and inclusive community."

Melvin Carter, who is running for mayor of St. Paul, said that as a black Christian, he found the image of a place of worship bombed during a time of prayer "one that I know, that we all should know, we cannot take lightly. … This has no place in Minnesota; this has no place in America; this has no place in our community."

Steve Hunegs, executive director of the Jewish Community Relations Council of Minnesota and the Dakotas, issued a statement reaffirming JCRC's "solidarity with the local Muslim community."

And Gov. Mark Dayton decried the incident and vowed to stay in close touch 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.

The Muslim American Society and the Minnesota chapter of the Council on American-Islamic Relations each offered a $10,000 reward for information leading to an arrest and conviction.

Staff writers Miguel Otárola and Jennifer Brooks contributed to this report.