The FBI is trying to determine why someone left an explosive device in a backpack in front of the police department of a small Colorado mountain town, prompting evacuations in the community’s main retail area.

A Nederland police detective found the bag outside the building when he began his shift Tuesday morning and brought it inside because he thought it was lost property. He opened it and found “suspicious articles” that were later determined to be an improvised explosive device, police Marshal Paul Carrill said.

The FBI, which had assumed control of the investigation by Wednesday afternoon, said the device was active but failed to detonate. Two robots searched the backpack, and multiple pieces were removed from the device and sent to a forensics lab.

“It’s difficult to interpret why someone would do this,” said Carrill, who declined to disclose if police have identified the backpack’s owner. “It is my hope and intent, for the health and safety of the community to get to the bottom of why.”

The police department is in the town’s main retail center, which includes a grocery store and multiple shops.

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.

(AP)