They said that Armstrong's blood values at the 2009 race showed clear blood manipulation consistent with two transfusions. Armstrong's red blood cell count suddenly went up at these points, even though the number of baby red blood cells did not.
Investigators said that was proof that he received a transfusion of mature red blood cells.
If Armstrong lied about the 2009 race, it could be to protect himself criminally, investigators said.
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Federal authorities looking to prosecute criminal cases will look back at the "last overt act" in which the crime was committed, they explained. If Armstrong doped in 2005 but not 2009, the statute of limitations may have expired on potential criminal activity.
The sources noted that there is no evidence right now that a criminal investigation will be reopened. Armstrong is facing at least three civil suits.
ABC News' Colleen Curry and Russell Goldman contributed to this report.