No immunity where A prosecutor procuring false testimony during an investigation and then introduced that same testimony at trial.

McGhee v. Pottawattamie  514 F.3d 739 (8th Cir. 2008)

Held: 

"We find immunity does not extend to the actions of a County Attorney who violates a person’s substantive due process rights by obtaining, manufacturing, coercing and fabricating evidence before filing formal charges, because this is not “a distinctly prosecutorial function.” The district court was correct in denying qualified immunity to Hrvol and Richter for their acts before the filing of formal charges."

NOTE: This case was accepted for review at the U.S. Supreme Court. The question presented for review:

"Whether a prosecutor may be subjected to a civil trial and potential damages for a wrongful conviction and incarceration where the prosecutor allegedly violated a criminal defendant’s “substantive due process” rights by procuring false testimony during the criminal investigation, and then introduced that same testimony against the criminal defendant at trial."

The case was ultimately dismissed by the [Supreme] Court when the defendants offered a settlement.  The briefs for this case are available at ScotusWIki