The Marin Humane Society has offered a $5,000 reward for information leading to the arrest of the person responsible for the deaths of three dogs in Mill Valley Tuesday afternoon.

The dogs belonging to neighbors of Mill Valley's Oakdale Avenue were taken for a morning walk by a dog walker on the Alta Trail above Marin City and died that afternoon after suffering high fevers and seizures.

The Humane Society is investigating whether the dogs died from something toxic they ate along the trail or whether the incident is a deliberate poisoning.

Dog owners are being advised to keep a close eye on their dogs in the Oakdale and Elinor avenues area of Mill Valley and along the Alta Trail.

Two of the dogs died after they were treated at Tamalpais Pet Hospital. A third was dead on arrival. Carol Anderson of Dog Dreams had walked the dogs along the trial, the Marin Humane Society said.

Dr. Penny Elliot did a necropsy on one of the dogs, a Lab, at the hospital. She said she found a pellet that resembled man made gopher bait in the stomach of that dog. The cause of death is pending toxicology tests which could take a week, she said.

Two of the dogs that died belonged to the same owner and the third belonged to the owner's neighbor, Elliot said. She said she's convinced the deaths are due to toxicity. The other four dogs on the walk did not become ill, she said.

"We don't know whether it was along the walk or in the Mill Valley neighborhood," she said this afternoon.

The Marin Humane Society is conducting necropsies on the other two dogs, Elliot said.