Who would try and harm animals at a popular East Bay dog park? That's what police are trying to find out after someone intentionally placed thousands of thumb tacks at a dog park in Hercules.

The tacks were found strewn throughout the Ohlone Dog Park, 190 Turquoise Drive, over the weekend, according to police.

One dog owner told NBC Bay Area a dog swallowed a tack placed at last week. Luckily, that animal was okay, although his owner had to pay hundreds of dollars for a vet bill.

A Hercules police officer first responded to the dog park at about noon Saturday based on reports of visitors to the park finding tacks that day and on Friday.

Tacks were found inside the park as well as in a creek nearby, according to police.

The tacks have apparently been reported at the park on at least one other occasion. 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.

"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.

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

Anyone who may have information about the case is asked to call Hercules police at (510) 799-8260.