The quaint little shop on the Upper East Side is New York City’s only store dedicated to French children’s books. But lately, the shop, La Librairie des Enfants, has earned a more sinister distinction: It has been the sometime home of Syko, a 98-pound white German shepherd with a penchant for eviscerating smaller dogs.

On Friday, Akiba Tripp was walking her seven-pound toy poodle, Baby, past the store when the owner opened the door and Syko lurched out, sank his teeth into Baby and broke her spine, Ms. Tripp said. Baby was euthanized that evening.

The attack followed two others in May in which Syko and his siblings injured three other dogs, their owners said. In recent months, the proprietor’s German shepherds — five of them, including Syko’s parents — had spent time in the basement and the main area of the store.

They terrorized people and dogs alike, according to several victims along with online reviews of the shop, which has an adjoining cafe.

Syko’s reign of terror has now apparently ended. His owner, Lynda Hudson, said that on Saturday, Syko and her other four German shepherds moved permanently to her new house in Westchester County and would no longer come to the store.

The news comes too late for Ms. Tripp, Baby’s heartbroken owner, a personal trainer who makes house calls to clients in the affluent neighborhood.

“My dog is dead,” she said. “Those dogs should have been away a long time before.”

Ms. Tripp said that when she went to the local precinct, the police told her they could not do anything about dog-on-dog violence, but she plans to file a civil suit against Ms. Hudson.

Ms. Hudson, 58, acknowledged that she was at her wit’s end with Syko. “I tried everything with this dog, sir,” she said. “I tried behaviorists, and training. He’s great with people but he does not like other dogs.”

Recent Yelp reviews capture some of the dissonance of a cute neighborhood fixture that was guarded by snarling menaces.

“Such a great little café,” reads one left on May 17.

Ten days later, another reviewer posted, “I was walking my 10 lbs Yorki and as we were passing by this shop, few German shepherds started barking, slamming to the glass windows covered in black plastic garbage bags. Then I see this nasty woman shouting in French, waving her hands to take my dog elsewhere!”

The first confirmed attack by one of Ms. Hudson’s dogs was on May 3. Julia Schafer, a psychologist who lives next to the bookstore, said that her husband walked out of the building with their collie mix, Tarsila, who was set upon by Syko and two other dogs — Syko’s siblings, Ms. Hudson said.

“The white one bit her and held onto her,” Ms. Schafer said. Tarsila required surgery. Ms. Schafer said Ms. Hudson reimbursed her $850 for the vet bills.

Four days later, a 22-pound Cavachon named Chloe and her Maltipoo friend Muppet were walking by when Ms. Hudson’s dogs struck again.

“Next thing I know this big white dog had my dog in her mouth,” said Chloe’s owner, Laurie Davis. “I’m screaming at the top of my lungs.”

Customers at the Third Avenue Ale House ran over and tried to break it up. One of the other dogs tore into Muppet’s face. Chloe needed surgery, too.

Ms. Hudson said she would pay the vet bills, which for Chloe came to over $6,000. But she balked at paying for Chloe’s day care during her recovery.

“I thought she is trying to milk me,” said Ms. Hudson, who was born in France and opened the bookstore in 2016. “I told her I am not a cow.”

Ms. Davis has sued Ms. Hudson in small claims court.

Ms. Davis said the police referred her to the city health department’s animal bite unit. She said she was later told by someone at the unit that they had interviewed Ms. Hudson, who told them the dogs had never attacked before and were moving out of the city in a few days, so they took no action.

The laws about aggressive dogs are complicated. According to a guide published by the New York State Bar Association, anyone can report a dog to the police or an animal control officer. A judge can then order the dog to be evaluated and muzzled, or removed, or euthanized.

Ms. Hudson said she sometimes kept her dogs in the basement of the store because a dog day care provider refused to accept them.

On Friday evening, Ms. Tripp and Baby, who was 2 years old, were walking to the 96th Street subway station. Baby went to the bathroom, and Ms. Tripp high-fived her as she always did. They walked by the Third Avenue Ale House. “She’s so cute!” diners called out.

As they walked by the bookstore, Ms. Tripp said, “all of a sudden the door opens, a woman walks out, and two large dogs just bum-rushed us.” Neither was muzzled. The white dog, Syko, “had my dog literally within his mouth.”

People ran over and tried to drag the dogs away. “Finally my dog was able to break free, but she couldn’t move — there was blood everywhere,” Ms. Tripp said.

Ms. Hudson walked Ms. Tripp and Baby to the vet. Ms. Tripp said the vet told her Baby would not walk again and recommended euthanizing her. Ms. Hudson says she offered to adopt and rehabilitate Baby and that Ms. Tripp turned her down.

Ms. Tripp had Baby put to sleep, and Ms. Hudson paid the bill.

Ms. Hudson said her business was already facing difficulties before her dog troubles. Last month, a supporter started a GoFundMe campaign to help the store pay rent and rehire laid-off employees.

Ms. Tripp’s birthday is next week. It will be two years since she received Baby as a gift.

Syko will turn 2 on Tuesday. Ms. Hudson said Monday she was considering putting him to sleep.

“There is really something wrong with Syko,” she said through tears, “and I don’t know what to do.”