Before Game 7 on Sunday, Byrd acknowledged using HGH after the San Francisco Chronicle reported he spent nearly $25,000 on the drug and syringes from 2002-05 - before HGH was banned by Major League Baseball.
The Indians lost 12-2 to the Red Sox, who broke open a 5-2 game with six runs in the eighth inning and advanced to the World Series for the second time in four years. Boston will host Game 1 against the Colorado Rockies on Wednesday night.
Mitchell, also a director for the AL champion Boston Red Sox, said his office was contacted by people accusing him of leaking information on Byrd to the media. The former Senate Majority Leader released a statement from his New York office to "correct that mistaken impression."
"Neither I nor any member of my investigative staff had anything whatsoever to do with the publication of the allegations about Mr. Byrd," the statement said. "We had no prior knowledge of those allegations, and we first learned of them, along with the rest of the public, through news accounts.
"Any information obtained in my investigation will not be made public until the report is released in the near future."
Mitchell has spent 1½ years investigating the use of performance-enhancing drugs like HGH in baseball and is expected to issue a report on his findings in November or December.
Byrd admitted to injecting HGH in the past, but says he has only done so under a doctor's supervision. According to the paper, the 36-year-old, who won two games in the postseason, bought HGH from an anti-aging clinic in Florida currently under federal investigation for illegally selling performance-enhancing drugs.
During a news conference outside Cleveland's clubhouse two hours before Game 7, Byrd said he has a pituitary tumor but would not confirm when he began using HGH or if he is still taking it.
Baseball banned the performance-enhancing drug in 2005, but does not test for it.
Byrd claims he has been working with major league baseball, a fact he said "shows I haven't tried to do anything behind anybody's back." He did not elaborate on how he is working with MLB.
Baseball officials said they were unaware of Byrd's use of HGH and planned to meet with the right-hander. If the Indians had advanced, the meeting likely would have taken place before the start of the World Series.
"Obviously, it's less pressing. He will be done in the very near future," said Rob Manfred, MLB's executive vice president for labor relations.
Byrd would need a therapeutic use exemption (TUE) to use any drug on baseball's banned list. However, MLB spokesman Pat Courtney said Sunday night in Boston that no player has ever been granted an exemption for HGH.
Byrd said he was disappointed by the timing of the Chronicle's report. He was concerned about it being a distraction for the Indians, who held a 3-1 lead in the best-of-seven ALCS before losing three straight.
Byrd apologized to his teammates and met with them individually before Game 7.
"Byrdman told us," starter Jake Westbrook said. "I didn't think it was a distraction. We know Paul and we know what the situation was and we know everything's going to be fine with that."
Byrd was not at Jacobs Field on Monday as Indians players cleaned out their lockers for the offseason.
His agent, Bo McKinnis, did not immediately return a phone message or e-mail.