HERCULES, Cali. (WCMH)–Police are trying to figure out who would have placed thousands of thumb tacks at a dog park in the Bay Area.

The Hercules Police Department said the tacks were found strewn throughout the Ohlone Dog Park over the weekend in this East Bay city.

One dog owner told NBC Bay Area that their dog swallowed a tack last week. Luckily, the dog was okay–but the vet bill set his owner back several hundred dollars.

A Hercules police officer first responded to the dog park at about noon Saturday. Visitors reported finding tacks that day and on Friday while in the park, and police say they found tacks there and in a nearby creek.

Dog owners say they have picked up between 3,000 and 5,000 of the tacks over the past few weeks and handed them over to police as evidence–and they say this isn’t the first time they’ve found them.

“They even found thumb tacks in the bowls for the water,” said Anthony Hayes, who brings his dog to the park.

There was no information available on potential suspects or witnesses, NBC Bay Area reported.

Hercules police have stepped up patrols in the area. The city is considering putting in cameras to try to catch whoever is responsible.