PORTLAND, Ore. - Over the past few decades, as an exotic pet trade boomed and Americans bought cute tiger cubs and baby monkeys, sanctuaries sprang up across the nation to take care of the animals that were abandoned when they reached adult-size or were no longer wanted.

The growth in both the number of wild cats as pets and the sanctuaries that rescued them has led to attacks.

Since 1990, more than 20 people have been killed by captive big wild cats at sanctuaries, zoos and private residences, more than 200 people have been mauled and 200-plus wild cats have escaped, according to one of the nation's largest wild cat sanctuaries.

The latest death is head keeper Renee Radziwon-Chapman, 36, who was killed by a cougar at an Oregon sanctuary last week.

Experts say that because sanctuaries are largely unregulated and anyone can open one, there are no uniform safety protocols. And over-confidence or human error can lead to tragic consequences even among the most experienced of caretakers.

"It's a risky business when you're dealing with dangerous wild animals. You can't leave any room for error," said Vernon Weir, director of the Nevada-based American Sanctuary Association which certifies sanctuaries.

For decades, exotic animals have been imported into the U.S. and openly bred for the pet trade. Despite new laws that limit the trade in some states, people can easily buy an African rodent, a chimpanzee, or a baby leopard at a flea market or over the Internet.

Experts estimate the U.S. exotic pet trade is a multibillion-dollar industry. Hundreds of sanctuaries have opened throughout the U.S.

About 80 sanctuaries currently house big cats, the International Fund for Animal Welfare says. Only a dozen of them are certified or verified by two certifying organizations, the American Sanctuary Association and the Global Federation of Animal Sanctuaries.

There's little governmental oversight. The U.S. Department of Agriculture, which enforces the federal Animal Welfare Act, licenses facilities that exhibit animals - whether domestic or wild - or do research. And the U.S. Fish and Wildlife Service tracks endangered species when they are bought and sold across state or U.S. borders and issues permits to facilities moving animals across state lines.

But neither agency keeps a tally on facilities or the total number of wild animals that are housed.

And no one sets rules for how sanctuaries operate.