GRAFTON — The dog was losing.

In an undercover video, men are shown hovering in a half-circle and goading the fight. On the floor, one pit bull can be seen shivering on its back, howling in pain as it’s repeatedly mauled by another dog.

Seasoned law enforcement officials and animal control officers cringed as they watched the brutal display. Some kept their gaze averted from the screen.

For four hours last month, more than 80 animal welfare professionals learned the language of blood sports in a lecture hall at Tufts University’s Cummings School of Veterinary Medicine. They joined veterinary students for a training that exposed an especially dark corner of American society.

“It is everywhere, whether you believe it or not,’’ said Terry Mills, director of blood sports investigations with the ­ASPCA. “If someone has a yard full of [pit bulls], he’s probably a dog fighter.’’

In 2016, the Department of Justice declared animal cruelty a “crime against society’’ rather than a crime against someone’s “property.’’ There is a known association between animal cruelty and other forms of interpersonal violence, said forensic veterinarian Dr. Martha Smith-Blackmore, the Tufts adjunct professor whose class hosted the training.

To help investigators, FBI officials have started tracking acts of animal cruelty as their own category of offense, among other felonies such as arson, burglary, assault, and homicide in the FBI’s criminal database. It used to be included with “All Other Offenses’’ in the Uniform Crime Reporting Program’s annual Crime in the United States report.

In cities, dogs are often kept in basements and attics. At professional dog fights, bets among wealthy entertainers, drug dealers, and athletes can go as high as a half-million dollars on the table.

Half of law enforcement officers polled by the American Society for the Prevention of Cruelty to Animals in 2015 said they have encountered dog fighting in their line of work. Only 23 percent said their department has the resources and training to investigate dog fighting cases.

At last month’s training, Boston, Brookline, and State Police officials sat next to animal control officers from Waltham to Gloucester to Coventry, R.I., and officials from the Department of Agriculture.

“We frequently find discarded, dead dogs that have multiple bite wounds,’’ said Mills. “They die from infection, kidney failure, or go into shock from blood loss. They die horrible, cold, lonely deaths.’’

There is a long history of dog fighting in New England, according to Lieutenant Alan B. Borgal, who attended the training and is director of law enforcement with the Animal Rescue League of Boston. He has worked with the organization for more than 40 years.

The number of people involved in dog fighting in the United States is in the tens of thousands. These estimates are based on injured dogs entering shelters and showing evidence of fighting and reports from underground publications on dog fighting.

Borgal has received tips out of Bristol County in Southeastern Massachusetts, and he has heard of dog fighting in parts of Cape Cod and New Hampshire, but people move, switch cars, and transport the dogs. They may breed and train locally and fight dogs in other states.

In the late 1990s and early 2000s, Borgal said, “Operation Dog Tag’’ was used by local enforcement to combat street dog fighting by tracking animals that were not properly registered.

An effort called “Operation Roadkill’’ out of Taunton in the early to mid-2000s shed light on motorcycle gangs that were organizing dog fights.

In 2009, two years after NFL star Michael Vick’s arrest brought dog fighting to the public eye, a female pit bull named Turtle was found lying on the side of Turtle Pond Parkway in Hyde Park covered in scars and open wounds. She was a bait dog. Bait dogs spend their lives being endlessly attacked to train other dogs to fight.

Turtle survived because of an anonymous phone call. Her recovery cost the Animal Rescue League of Boston nearly $10,000 in veterinary fees.

She became a local ambassador to draw awareness to dog fighting.

As often happens, her abuser was never found.

“Pit bulls are one of the most abused animals I’ve seen,’’ said Borgal. “They also make up most of the dogs in our shelter.’’

In 2015 in Bridgewater, two dogs were found dead. Police thought both had been shot. Smith-Blackmore said what authorities thought were bullet holes were actually bite wounds.

No suspects were arrested in that case either.

“Often, when there’s a dead dog found floating in the harbor, these cases go unsolved,’’ Smith-Blackmore said. “There’s the potential for them to be solved if we have the right people in the right place with the right investigators.’’

During the training, pictures of puppies in large weighted collars were shown next to images of adult dogs covered in scar tissue. Mills brought in tools of the trade. One was a break stick, which is forced into a dog’s mouth to stop it from latching on to its opponent. Then, there was a set of cables used to shock dogs. There were photos of dogs with limbs amputated with no anesthesia, dogs whose ears were trimmed so they wouldn’t be tattered in a fight, dogs whose lips were cut back so they wouldn’t bite through their own mouth. Many lost teeth during fights.

At a young age, pit bulls are tested to see how willing they are to attack one another, Mills said. The dogs that squealed or lost during these practices were put down in horrific ways.

These dogs don’t die in a bout. They die after, at the hands of the one that bred them to fight. “A dog fighter is not going to feed a dog that doesn’t fight,’’ Mills said. “If that dog’s not game, it’s gone.’’

In dog yards, rusted barrels are used as dog houses. There are stakes in the ground and heavy metal chains. Despite the abuse, conditioning, and fights, most pit bulls were taught to be aggressive toward other dogs but not humans, Mills found. In some cases, dogs taken from rings wouldn’t know how to play fetch. They had never been socialized.

“These dogs, they just look at you,’’ Mills said. “They don’t know what to do. They never played with a toy.’’

Cristela Guerra can be reached at cristela.guerra@globe.com.