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SANTA CLARA, Calif. — As Broncos quarterback Peyton Manning prepares for Sunday’s Super Bowl against the Carolina Panthers, he is still dealing with allegations he used human growth hormone to help him extend his career.

The charges stem from a controversial documentary by Al Jazeera. But the TV network wasn’t the only one investigating the allegations, according to The Washington Post.

The report says Manning and his lawyers knew the documentary was in the works so they hired private investigators to look into the charges.

The investigators were quick to target Charles Sly, who was making the allegations against Manning in the undercover video. Sly, a former pharmacy intern, said The Guyer Institute in Indianapolis shipped HGH to Manning’s wife Ashley in 2011.

HGH is banned by the NFL and it is illegal to prescribe except for a few diseases.

The Washington Post article says five days before the documentary aired in late December, the investigators showed up at the home of Sly’s parents, Randall and Judith, in Brownsburg, Ind. They were dressed in black and said they were looking for Sly.

The family called 911, not sure who was at their front door. The Slys talked with the private investigators, Brian Bauer and Ben Ford, The Post reported. Bauer and Ford returned the next day to speak with Charles Sly, asking about his background and relationships with the athletes named in the Al Jazeera report.

In the meantime, Manning’s lawyers began a probe of The Guyer Institute the sports medicine clinic in Indianapolis that supposedly supplied Manning and other big names in sports with HGH.

The day after Manning’s investigators came to his home a second time,  Sly’s father made his son record a statement that denied any of the charges against the quarterback, saying he made it up.  Manning has vehemently denied all of the allegations, calling the report “garbage.” The NFL is still investigating.