NEDERLAND, Colo. — Multiple buildings were evacuated after someone left an explosive device in a backpack in front of a Colorado police department.

A Nederland detective found a backpack outside the building when he began his shift Tuesday morning and brought it inside, believing it was lost property, police Marshal Paul Carrill said. He opened it and noticed “suspicious articles” that investigators later determined to be an improvised explosive device, Carrill said. He would not elaborate.

Two robots searched the backpack, and multiple pieces were removed from the device.

Carrill would not say whether police have identified the backpack’s owner. Investigators have not determined a motive.

“It’s difficult to interpret why someone would do this,” Carrill said. “It is my hope and intent, for the health and safety of the community to get to the bottom of why.”

The FBI, which assumed control of the investigation by Wednesday afternoon, said the improvised explosive device was active but failed to detonate.

The police department is in the mountain town’s main retail complex, which includes a grocery store and multiple shops. Those businesses were evacuated. Carrill urged the public to come forward with information because “the individual or individuals who may have assembled this could be a risk to their neighbors.”

Nederland is about 15 miles southwest of Boulder.