Napa police Sgt. Debbie Peecook said the reward for information leading to the arrest and conviction of the person or people responsible is currently $4,500.
The Humane Society of Napa also is accepting pledges toward the reward, executive director Jane Albert said.
"We're just devastated by this," she said this morning.
Napa resident Sheri Smith's two retired greyhounds and a German shepherd were found dead in her backyard in Browns Valley on Oct. 29.
Napa police officer Ryan Cole's two German Shorthaired Pointers and his police dog partner also were poisoned Nov. 3 with snail bait thrown in his backyard. One of Pointers died, Peecook said.
Peecook said the tan, pellet-like snail bait was in cereal-size boxes and the tops were found open.
It's unclear whether the dogs opened the box, but it's logical to suspect the two cases are related, Peecook said.
The person responsible could face four felony counts of animal cruelty leading to death and a misdemeanor charge of attempting to injure or kill a police dog, Peecook said. The felony charge carries a three-year prison term.
Anyone with information is asked to call detective Mike Walund at (707) 257-9223 or Cmdr. Steve Potter at (707) 258-7882.