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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, an incident that prompted a separate local sheriff's office to temporarily shut down Thursday when someone stepped forward to offer information about the case.

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.

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Map showing location of Nederland, Colorado. Google Maps

"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."

The Boulder County Sheriff's Office in Boulder was temporarily shut down Thursday morning after a person came forward to provide information about the incident.

Nothing threatening was found in the vehicle and the sheriff's office was reopened a short time later, said FBI spokeswoman Deborah Sherman.

She declined to comment on whether the person had any useful information about the device left in a backpack Tuesday outside the Nederland, Colorado police department.

Nederland is about 15 miles southwest of Boulder.