Federal investigators said an explosion at a mosque in Minnesota was caused by an improvised explosive device as they appealed for help in finding the culprits.
The blast happened at around 5am at the Dar Al Farooq Islamic Centre in Bloomington.
Wellwishers have raised almost US$10,000 (Dh36,700) to help with repairs.
The nature of the attack, suggesting a hate crime, has provoked an angry backlash on social media among activists wanting to know why it has not been classed as terrorism.
Windows of the imam's office at the mosque were shattered but no one was hurt.
About 20 worshippers had already arrived for the day’s first prayers.
One raced outside to see what had happened after the blast echoed through the mosque, setting off fire alarms, according to Mohamed Omar, the centre’s executive director.
“He saw a truck fleeing form the parking lot at very high speed,” he said, adding that smoke poured from the broken window.
Automatic sprinklers doused a fire inside the imam’s office, leaving a mess of damaged and wet furniture.
Mr Omar said the mosque, which primarily serves people from the area's large Somali community, occasionally receives threatening calls and emails.
The site was once used as a school but was converted into a mosque and community centre in 2011
Some nearby residents objected and there have been occasional complaints about cars parking in the area although Saturday’s attack was the first such violent incident.
Yasir Abdalrahman, who has been worshipping at the mosque for two years, told the Minneapolis Star Tribune he was shocked by the attack.
“We came to this country for the same reason everyone else came here — freedom to worship,” he said. “And that freedom is under threat. Every other American should be insulted by this.”
The bombing comes at a time of heightened anxiety for America’s Muslims. There has been a rise in reported anti-Muslim crimes, from arson attacks at mosques to harassment of women wearing head scarves, coinciding with the election of a president who has tried to ban travellers arriving from six mainly Muslim nations.
Vandals recently attacked the nearby Al Maghfirah cemetery in Castle Rock Township, spray painting profanities and swastikas.
Against that background, some activists have questioned why investigators have been slow to label the attack a “hate crime” and why some news organisations have described the mosque attack without using the term “terror” or “terrorists”.
They accused newspapers such as The New York Times of double standards for headlines such as "Minnesota Mosque Hit by Early-Morning Explosion", saying attacks carried out by Muslims would have been described in less neutral terms.
"Hi @nytimes: Let me correct that headline for you. "Terrorist attacks Muslims in Minnesota, bombs their place of worship," wrote Simran Jeet Singh on Twitter,
Richard Thornton, the FBI special agent heading the investigation, appealed for anyone with information to come forward.
“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.”
He said investigators had recovered parts from an improvised explosive device and were trying to understand how it had been constructed.
They were now sifting through security camera footage and mobile-phone video as they tried to establish whether it was hate crime and who was behind it, he added.
The mosque is offering a $10,000 reward for information that leads to an arrest or conviction.
The region’s religious leaders offered messages of support at an interfaith news conference hours after the latest attack.
Curtiss DeYoung, chief executive of the Minnesota Council of Churches, said: “We are here showing solidarity and support for Muslims not only in this centre but for Muslims across our state and the country who are under attack.”
He said the state’s million Protestants offered their sympathy
“An attack on a mosque is an attack on a synagogue is an attack on a church is an attack on all faith communities,” he said. “So we stand with you.”
An online GoFundMe appeal had raised almost $10,000 in its first 16 hours towards a target of $95,000 to repair the damage and support the mosque’s work.