The Humane Society of the United States is threatening a lawsuit against Amazon.com Inc. (AMZN), saying the online retailer is violating federal law by selling two magazines that promote cockfighting.

The Humane Society sent a letter Tuesday to the retailer, detailing its intention to begin legal action unless the magazines — "The Feathered Warrior" and "The Gamecock" — are removed immediately from the site. Both are mailed from western Arkansas.

The group cited the Animal Welfare Act, which says it is unlawful to use the "mail service or the U.S. Postal Service" to promote "an animal-fighting venture except as performed outside the states of the United States."

"That's their claim, and we don't agree with their claim, so we're going to continue to make these titles available," Amazon.com spokeswoman Patty Smith said Tuesday. "It's up to the customer to determine what they feel is appropriate for them to purchase."

Cockfighting is legal only in Louisiana and New Mexico.

In April, The Humane Society asked the Postal Service to stop allowing the magazines to be sent through the mail, citing the act and postal regulations. Last year, the group asked Amazon.com to stop selling the magazines, but did not get a response.

The Humane Society's Ann Chynoweth said it would be difficult to stop a magazine from being published — but her group hopes to slow circulation when possible.

Last month, Amazon.com said it would pull a DVD featuring violent pit bull fights that unleashed protests against the distributor and several online merchants that had been peddling the video.

Verna Dowd, editor of "The Feathered Warrior," said she wasn't concerned about The Humane Society's threatened lawsuit.

"I don't know anything about it," she said. "I didn't even know they posted my magazine on Amazon."