MEXICO CITY (AP) -- Police scoured a hilly urban park for feral dogs and tested dozens of captured animals on Tuesday in a hunt for those responsible for four fatal maulings that have set off a fierce debate about how to handle the thousands of stray dogs that roam this massive city.

Authorities have captured 25 dogs near the scene of the attacks in the capital's poor Iztapalapa district, but rather than calm residents, photos of the forlorn dogs brought a wave of sympathy for the animals, doubts about their involvement in the killings and debate about government handling of the stray dog problem.

Activists started an online campaign protesting the dogs' innocence and calling for authorities not to euthanize them. Tens of thousands of dogs are euthanized each year in Mexico if they are captured by animal control officers and not claimed within 72 hours. Many people re-posted the images of the dogs staring sadly from behind bars at an animal shelter.

The hashtag for the campaign became the top trending topic on Twitter in Mexico by midday Tuesday, with some users furiously accusing the authorities of cruelty to animals and others sarcastically calling the dogs "political prisoners" and mocking the fuss over the fate of the animals.

Officials said they were testing the captured dogs' fur for blood, and examining their stomach contents to determine if they were the killers of the four people whose bodies were found covered in dog bites in two separate incidents in recent days.

Neighbors of the Cerro de la Estrella park found the bodies of a 26-year-old woman and a 1-year-old child in the area on Dec. 29, authorities said. The woman, Shunashi Mendoza, was missing her left arm, and prosecutors said that both she and the boy had bled to death.

Then on Saturday visitors to the park found the bodies of Alejandra Ruiz, 15, and her boyfriend Samuel Martinez, 16, who had gone to the park Saturday afternoon and were found dead from blood loss. The girl called her sister Diana Ruiz at around 7 p.m. pleading for help, the sister told Milenio Television.

Mexico City prosecutors said that due to the gravity of the wounds they believed at least 10 dogs were involved in each attack.

Dozens of officers returned to the park Tuesday to capture more of the feral dogs, which live in caves and hollows in the area.

"Officer, you're hunting for dogs again, but don't you already supposedly have the 25 killers in custody?" shouted Liliana Hernandez, a psychologist and self-described street dog rights activist who lives near the park.

Hernandez said many people let their dogs out during the day because their cinderblock homes are too small to keep them inside. Residents of their neighborhood started running frantically to collect their dogs when police began seizing strays Monday night, she said.

The furor has forced a public response from Mexico City Mayor Miguel Angel Mancera, who called for animal-rights groups to help study the guilt or innocence of the 25 dogs, and the broader effort to reduce the number of street dogs in Mexico City.

"We're not taking any decision. The dogs are in a shelter and we have to check on their health," he told reporters after a midday press conference.

He also said the government would launch a new program to spay and neuter dogs, sending 25 mobile surgical units to neighborhoods where residents would be encouraged to take advantage of free sterilization for their pets.

Mexico City Attorney General Rodolfo Rio held a news conference late Tuesday to reiterate the victims bled to death after dogs mauled them.

"We didn't find any other type of wound different from those produced by dogs," Rios said.

Antemio Maya, president of the Street Dog Protection association, said estimates for the overall number of dogs in the city of nearly 9 million people range from 1.2 million to 3 million animals but there are no reliable figures for the number of street dogs. That's partly because many of the animals are pets released to wander the streets during the day, and then return home when their owners come back from work.

He said many people in Mexico City treat dogs as disposable, buying one for their children for Christmas and abandoning it in a park or suburb when the responsibility becomes overwhelming.

On Tuesday, people seem unfazed by the killings and went by their daily routine as usual with children coming back from school and venturing into the tree-covered grounds, housewives crossing the park with their groceries and men on horses checking maize fields.

Residents who live around the park and relatives of the 16-year-old high school boy who was killed refused to accept that dogs killed the youngest member of their family.

"It's not logical. I mean the dogs are small, they are domestic dogs. We don't believe what the prosecutor's office is telling us," brother Enrique Martinez told The Associated Press.

Martinez said police told the family the couple was perhaps carrying a fish fillet which alerted the dogs. He said identifying the body was a horrible experience for him and his loved ones.

Fatal maulings of humans by dogs are extremely rare, but Dr. Sophia Yin, executive board member of the American Veterinary Society of Animal Behavior, said they can happen, particularly when dogs that are used to being around people are moving in large groups, and people shout or run in fear when accosted by the animals.

"It's hard to tell people, stand completely still and relax, don't turn your back. Almost anything else you do can get the dog more excited," Yin said. "The more animals involved, the higher the excitement is."

Jose Luis Rodriguez, driver for a micro-bus service that travels through the park, said he had recently encountered a pack of 10 feral dogs that had growled at his father.

"There are a lot of dogs around here. We give them something to eat and they go away," he said. "People come and throw their dogs away in the park, abandon them. There are too many dogs around."

A veterinarian at the Iztapalapa shelter where the 25 dogs were being held said it appeared that at least one of the captured dogs had been a pet.

"A family came and recognized a dog but we couldn't give him to them because investigators have to decide what to do," he said on condition of anonymity because he was not authorized to speak to the press.

Rios said authorities will keep the trapped dogs until investigators determine whether they were involved in the attacks. He said people who can prove one of those dogs belongs to them can take the animal home. He said the rest will be put up for adoption.

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Adriana Gomez Licon is on Twitter http://twitter.com/agomezlicon

Michael Weissenstein on Twitter: http://twitter.com/mweissenstein