NEDERLAND, Colo. — This Colorado mountain refuge of hippies and artists has long thrived on close ties among neighbors and a welcoming embrace of outsiders.

But a backpack containing a homemade explosive, dropped on the doorstep of the town’s five-officer police department, has left many of these take-it-as-it-comes residents shaken.

Authorities on Thursday were still investigating to find who placed the device in front of the police station in Nederland’s only strip mall, within sight of a nature center, a brewery, several legal pot shops and a train car once used in Buffalo Bill’s Wild West show that now serves coffee and burritos.

The shopping center was evacuated on Tuesday while robots searched the backpack and investigators detonated the explosive with a startling bang.

A detective discovered the backpack when his shift began early Tuesday and brought it inside the stationhouse, believing it was lost property, officials said. He opened it to find items investigators later determined were an improvised explosive device.

The FBI said it was an “active” explosive that failed to detonate, but Denver field division spokeswoman Deborah Sherman would not elaborate Thursday. She said investigators had not pinpointed a motive and urged the public to call with tips.

The sheriff’s office in Boulder was closed Thursday morning as authorities searched the vehicle of a person who came forward to provide information. They found nothing threatening and the office reopened a short time later.

“To my knowledge this has never happened before here in Nederland,” said John Scarffe, who works in a visitor’s center and writes for the Mountain-Ear newspaper. “People are pretty surprised, we don’t really know who might have been involved. Most of what is in Nederland is within walking distance. Had there been an explosion, there is a good possibility people right here would have been hurt.”

The swarm of police officers was an unusual sight in this mountain-ringed town of 1,500 people southwest of Boulder best known for its love of legal marijuana and its annual celebration of a frozen corpse that draws tens of thousands of revelers.

Residents have recently complained about homeless campers and wanderers who live off the land. Two people camping near an area popular with transients attracted to the laid-back community were charged this summer with accidentally sparking a fire that destroyed eight homes by not properly putting out their campfire.

But if there is anti-police sentiment, longtime residents said it’s hard to immediately spot.

“It’s a tiny little mountain town where there’s a lot of weird folks,” said Robert Spratford, who works in the Train Cars coffee shop not far from the police station.

Town administrator Alisha Reis called the bomb scare bewildering.

“It’s troubling to us. People are hoping to understand why this happened, who is responsible for it and to understand a little better about what was behind it,” she said. “And, why Nederland?”