(CNN) -- An estimated 40,000 people in the United States are involved in professional dogfighting, an illegal blood sport with fight purses as high as $100,000.

The latest accusations against Atlanta Falcons quarterback Michael Vick and three other people highlight the problem. They are accused in an indictment that describes dogs being routinely executed if they didn't fight fiercely.

The indictment was handed down Tuesday by a federal grand jury in Richmond, Virginia.

The nightmare of dogfighting is growing, according to the Humane Society of the United States.

John Goodwin, an expert on animal fighting with the Humane Society, says there are an estimated 40,000 professional dogfighters in the United States, involved in putting on fights and buying and selling fighting dogs. Watch what goes on at a high-stakes dog fight »

But, Goodwin adds, there could be as many as 100,000 additional people involved in "streetfighting" -- informal dogfighting, often involving young people in gangs.

"It's far more pervasive than people think and it's definitely been on the upswing in the past five to 10 years," he told CNN. See how dogfighters operate and have their own language »

Statistics from animal shelters give another indicator of the rise in dogfighting, Goodwin said. Fifteen years ago, 2 to 3 percent of the dogs coming into animal shelters were pit bulls; now, he said, pit bulls make up about a third. At one shelter in Jersey City, New Jersey, Goodwin said, the figure is 65 percent, with 20 percent of them showing the scars that indicate they have been fighting dogs.

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A database run by animal advocacy group Pet-abuse.com, which collects reports of animal abuse, shows reports of dogfighting cases increased from 16 in 2000 to 127 in 2006. The group has found 74 cases reported so far this year.

Dogfighting is illegal in all 50 states. It's a misdemeanor in Idaho and Wyoming, and a felony everywhere else. But in some states where dogfighting is a felony, it's still perfectly legal to own a fighting dog or be a spectator at a dogfight. See where it's legal to be a spectator »

A bill signed by President Bush in May made the federal law against dogfighting tougher, by strengthening penalties to felony level. The law bans interstate commerce, import and export related to animal fighting activities. Violators can now be sentenced to three years in jail and a $250,000 fine. Previously the maximum sentence was a year in jail.

Despite the laws, dogfighting is big business. Goodwin said it's impossible to estimate the amount of money involved, but the purse for a top-level professional fight could be $100,000.

"There are about a dozen underground dogfighting magazines, and about half a dozen ... registries that are exclusively used by either dogfighters or people that are fighting dog enthusiasts," Goodwin said. "You have an organized infrastructure for what is a criminal industry." E-mail to a friend

All About Dogs • Humane Society of the United States