Someone outraged by the slaughter of 100 healthy sled dogs in Whistler, B.C. has threatened to take vigilante action, prompting police to open another criminal investigation.

RCMP Staff Sgt. Steve LeClair told that police are investigating a threatening email sent in connection to the dog cull that has prompted the anger of people across Canada and the U.S.

"The threats are serious in nature," he said. He would not elaborate on the content of the threat or who received it.

LeClair said police have not determined if the threat is valid, but those targeted have taken protective action.

"It's hard to say if it's someone that's venting or someone intent on carrying out that threat," he said.

News of the mass killing came to light when WorkSafe BC documents revealed that dozens of dogs were massacred in April 2010 because of a "slow winter season" after the Winter Olympics.

The general manager of Howling Dog Tours, a company owned by Outdoor Adventures Whistler, filed a claim for post-traumatic stress after shooting the dogs to death. Some of the dogs survived shots to the head, and had to be chased down as they ran around with parts of their faces missing.

In response to the news, animal lovers have taken to social networking and online petitions to express their disgust. By early Tuesday afternoon, a Facebook fan page called "Boycotting Outdoor Adventures in BC, Whistler" had gathered more than 18,000 fans.

But LeClair said some of the thoughts expressed on social networking pages veered into vigilantism, and were taken down by site administrators.

Mounties are looking into the slaughter as part of an SPCA-led investigation and are urging calm until the investigation is complete.

"We just want the public to be patient and know that we'll get to the truth of the matter," LeClair said.

Criminal charges for animal cruelty are possible against the worker who killed the dogs, but the B.C. SPCA said Tuesday that the business owners are unlikely to face charges.

Outdoor Adventures Whistler denies having operational control of Howling Dogs until after the mass dog cull.

Unhappy dog owners have planned a "Barking Mad" march to protest the killings at West Vancouver's Ambleside Park on Sunday.