At least six meatballs laced with rat poison were discovered at a park in Colorado since Monday, the Daily Camera reported.

The disturbing investigation began at the beginning of the week after Uli Bromme, a local dog owner, called the Boulder County Sheriff's Office to report that her puppy ate something suspicious.

In addition to Bromme's dog, at least two other dogs have ingested the poisonous meatballs.

"Everyone in this neighborhood is shocked," Bromme told the Daily Camera. "We all know each other; the dogs know each other. That's where we go to socialize, and for our dogs to socialize."

After Bromme's puppy ate one, she was able to pick up other meatballs before any other dogs got to them. She told the local paper that it looked like hamburger meat with blue and green pellets. After rushing Amos to the veterinarian, he was given an injection that caused him to vomit the food, which tested positive for rat poision.

Sara Spensieri, an animal control specialist, said all dogs affected are recovering but noted the sheriff's office has no leads.

Additionally, all dogs that ate the meatballs will be monitored for a few days since symptoms of poisoning can take two or three days to show, according to Matt Booth of Alpenglow Veterinarian Clinic in Boulder.

Booth said pet owners should look for signs of lethargy, decreased appetite, pale gums, difficulty breathing, and unexplained bleeding or bruising.

Anyone with information on the investigation is asked to call the Boulder County Sheriff's Office at (303) 441-4444.