The athletes helped the FBI spot fake signatures during a two-year undercover FBI investigation known as "Operation Bullpen," ABC's "20/20" reported.
Gwynn told "20/20" he went to the FBI three years ago after discovering the San Diego Padres' own gift shop was victimized by suppliers of fake baseball memorabilia.
"As I'm walking in, I'm looking in the window, and they had a nice display of Padre baseballs, and 8x10s with other players," Gwynn said. "And I'm looking, and I started looking at the signatures. I mean, every one in there every single one was a forgery."
The FBI then got Sosa and McGwire involved as well, the report said, and it has several tractor trailer loads of forgeries from around the country.
According to "20/20," authorities estimate faked sports memorabilia have become a $500 million scam in recent years.
"There are so many unsuspecting people out there who are getting ripped off, they probably don't know if they're getting ripped off," Gwynn said. "They have got balls and pictures and bats sitting on shelves and in cabinets and it hasn't probably dawned on them that it might not be real."
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