A resident found the package — wrapped in plastic and held together by a rubber band — on Tuesday and then contacted local television station KAKE-TV. The station subsequently called police.
The package contains what appears to be the driver's license of Nancy Fox, who was found tied up and strangled in her home on Dec. 8, 1977. BTK's voice was captured on tape when he called a dispatcher to report Fox's murder.
Wichita police officials released a statement saying they had submitted the contents of the package to the FBI for authentication.
The Wichita Eagle interviewed a woman who worked with Fox. She said the Fox driver's license, which she saw on TV, appeared to be authentic. The woman said she recognized Fox's photo and signature on the license.
"I'm not a handwriting expert, but that was it [Fox's signature]. My gut feeling is it's for real," she told the newspaper.
The BTK serial killer, whose nickname stands for "Bind, Torture, Kill," is linked to eight unsolved murders from 1974 to 1986. He resurfaced in March with letters to media and police.
KAKE-TV showed film footage of the contents of a package, including an unidentified wrapped metallic object and blurred images of three pages purported to be a list of chapters of a book entitled "The BTK Story."
In a 1978 letter, the killer also included a poem titled "Oh! Death to Nancy" that had striking similarities with the lyrics of the old folk song "Oh Death."
Police said they have received close to 5,000 tips from the public since BTK resurfaced in March.