The FBI has offered a reward for information about a potentially lethal bomb found in a backpack along the route of a Martin Luther King Day parade in the US.
The discovery in Spokane, Washington, before Monday's parade for the murdered civil rights leader raised the possibility of a racial motive in a region that has been home to the white supremacist Aryan Nations.
"The confluence of the holiday, the march and the device is inescapable," said Frank Harrill, special agent in charge of the Spokane FBI office. "But we are not at the point where we can draw any particular motive."
He called the planting of the bomb an act of domestic terrorism that was clearly designed to advance a political or social agenda.
The suspicious backpack was spotted by three city employees about an hour before the parade was to start, Mr Harrill said. They looked inside, saw wires and immediately raised the alarm.
The parade route was changed to avoid the device and a bomb disposal unit disabled it without incident, he said.
Mr Harrill would not release details about the device, other than to call it a functional bomb that could have caused multiple casualties.
"The potential for injury and death were clearly present," he said.
The FBI received no warnings in advance and did not have a suspect, Mr Harrill said. No-one has claimed responsibility for planting the bomb.
The agency decided to appeal to the public for information and offered the £13,000 reward for information leading to an arrest and conviction.