Title 18, United States Code, Section 1001 makes it a crime to: 1) knowingly and willfully; 2) make any materially false, fictitious or fraudulent statement or representation; 3) in any matter within the jurisdiction of the executive, legislative or judicial branch of the United States. Your lie does not even have to be made directly to an employee of the national government as long as it is "within the jurisdiction" of the ever expanding federal bureaucracy. Though the falsehood must be "material" this requirement is met if the statement has the "natural tendency to influence or [is] capable of influencing, the decision of the decisionmaking body to which it is addressed."
(Ironically, the government lies to us all the time, but there are apparently no penalties for that.)
That article gives a ton of good reasons to flat out refuse to talk about anything of substance to any agent, ever, without your attorney present.
I would highly recommend reading it. Even if you don't think you're personally the target of an investigation, the way these things go, anyone that they can catch in the net is a potential target, so you should be nervous - it always looks better to indict N+1 people than N, and if you talk to the investigators at all without a lawyer present, you greatly increase your chances of being that +1, even if you truly and honestly haven't done anything wrong.
How to Avoid Going to Jail under 18 U.S.C. Section 1001 for Lying to Government Agents
http://library.findlaw.com/2004/May/11/147945.html
Excerpt:
Title 18, United States Code, Section 1001 makes it a crime to: 1) knowingly and willfully; 2) make any materially false, fictitious or fraudulent statement or representation; 3) in any matter within the jurisdiction of the executive, legislative or judicial branch of the United States. Your lie does not even have to be made directly to an employee of the national government as long as it is "within the jurisdiction" of the ever expanding federal bureaucracy. Though the falsehood must be "material" this requirement is met if the statement has the "natural tendency to influence or [is] capable of influencing, the decision of the decisionmaking body to which it is addressed."
(Ironically, the government lies to us all the time, but there are apparently no penalties for that.)