LONGMONT, Colo. -- Longmont police are warning neighbors to be on the lookout after authorities found a series of homemade explosive devices on Friday.

Police and ATF agents found several cartridge cases filled with gunpowder in a Longmont clinic parking lot.

Even though the devices were small, police said if someone stepped on or drove over one, it could have caused serious injury.

Officers said in April they found similar explosives in the same parking lot near Mountain View Avenue and Tulip Street.

Neighbors said they have been hearing loud bangs, and just over two weeks ago someone set off a homemade bomb in a neighbor's pickup truck.

Officers said they believe the incidents are all related.

"To be honest with you, I'm pretty scared," said Leon Pinley, the owner of the truck a bomb went off in. "I don't know what's going on (or) who's doing this."

Police said so far no one has been seriously injured. They're asking anyone with information to come forward and call the Longmont Police Department at 303-651-8555.