The dogs - Shih Tzus, Lhasa apsos and Chihuahuas - were packed into a half-dozen small crates, police said.
"They were about the size of a cat crate," police Officer Charles Flahive said. "There were three or four dogs in each one."
More than one container had as many as five dogs in it, said Animal Control Officer Kelly Demers.
"It was cruel to have them all stuffed in crates," said Demers, who also is a police officer.
A woman called police at about 10:30 a.m. yesterday after she heard two women talking in Wal-Mart about having dead dogs in their car. The two were shopping for some kind of dog medication when the caller overheard their conversation, police said.
The caller, a police dispatcher in the Boston area, met Flahive outside the store just as the pair were leaving. Amy Moolic, 32, of Dracut, Mass., brought Flahive and Demers to her Jeep Cherokee. Once the officers questioned her, she became confrontational, police said.
"The driver of the car was extremely belligerent and creating a disturbance," Flahive said.
Moolic was charged with disorderly conduct and interfering with a police officer, both misdemeanors.
"The story she gave us was that she traveled to Somerville, Maine, and rescued these dogs from a puppy mill," Flahive said. "The smell and conditions inside the car were terrible. They were all shaking and scared. They were all in tough condition."
Demers contacted animal cruelty officials in Massachusetts and Maine to investigate Moolic's claim. He said she could face animal cruelty charges of storing the dogs in such tight quarters.
One of the dogs bit Demers on the chin when he was removing it from the Jeep.
"They're a feisty lot," he said.
Demers said he believes the dead dogs were a mother and two puppies. He was uncertain why or how they died.
Moolic told police she had rescued 12 of the dogs from a Maine puppy mill, police said. The other 10, she said, were hers and just along for the 150-mile ride to central Maine.
Police took all the dogs into protective custody. Moolic may be able to reclaim some of them, Demers said, and others will be offered for adoption.
The dogs Moolic claimed to have owned before yesterday were clean and in good health, he said. But the other animals, most of them about 10 years old, were in rougher shape, with dirty, matted fur.
Moolic was released on $500 bail shortly after her arrest.
"Right now, we don't think there was any abuse going on," said Sgt. James Chase. "We've taken the dogs into protective custody."
The dogs are being kept at the Salem Animal Rescue League.