All federal prosecution is done under the Justice Department, which is part of the executive branch-- as is the IRS, NSA, FBI, etc.
So, if the president is fine with them lying to congress, there isn't anything congress can do short of impeaching and removing the president. Good luck with that.
I remember during the Bush years where administration leaders were flat out ignoring congressional subpoenas, and there was nothing the investigating committee members could do to force them to show up since the justice department was instructed not to act.
