An explosive that shattered windows and damaged a room at a Minnesota mosque was "a criminal act of terrorism", the state's governor Mark Dayton has said.
Congregants were arriving for prayer early on Saturday when the explosion went off at the Dar Al Farooq Islamic Center in a Minneapolis suburb.
The FBI are investigating what Mr Dayton called "a terrible, dastardly, cowardly" act.
They said the blast was caused by an improvised explosive device.
No one was injured in Saturday's attack at 0500 local time (0900 GMT), but the imam's office was heavily damaged.
As many as 20 people were inside at the time of the explosion.
Mohamed Omar, the executive director of the mosque, told CBS News a member of the congregation saw a truck flee the centre's car park at high speed after the blast.
The office's windows were shattered either by an explosive or another object was thrown inside, authorities said.
"The destruction done to this sacred site is just unthinkable, unforgivable. I hope and pray the perpetrator will be caught and prosecuted to the full extent of the law," Mr Dayton said at a news conference outside the mosque, located in Bloomington, Minnesota.
He was joined by state Representative Ilhan Omar, the first Somali-American legislator in the US, and US Representative Keith Ellison, the first Muslim elected to Congress.
"What makes Minnesota unlike no other [state] is how we accept and love members of our community, no matter the religion they practise, the language they speak, or where they come from," Mr Ellison said.
He added "hate is becoming too prevalent in our society" and that it must be countered "with love and the values we hold so dear".
The Minnesota chapter of the Council on American-Islamic Relations (CAIR) is offering a $10,000 (£7,700) reward for information leading to the arrest of those responsible for the attack.
The organisation has also urged mosques and Islamic centres across the country to increase security in the wake of Saturday's attack.
The Muslim American Society of Minnesota also said it would offer a $10,000 reward for information on the attacker.
A fundraising page created to help pay for costs to repair the mosque raised more than $40,000 as of Monday morning.
An estimated 3.3 million Muslims live in the US, around 1% of the population, according to a recent Pew Research Center report.
Minnesota is home to the largest population of Somalis living in the US and the Dar Al Farooq Islamic Center largely serves people from the Somali community.
Minnesota authorities reported a record high of 14 anti-Muslim bias incidents in 2016, The Star Tribune reported.