Section 1512(a) proscribes conduct intentionally undertaken, section 1512(b) proscribes conduct "knowingly" undertaken, and section 1512(c) proscribes conduct "intentionally" undertaken. A state of mind commonly referred to as "general intent" was prescribed by the use of the terms "knowingly" and "intentionally." General intent means that the person is aware of the nature of his/her conduct and those circumstances incident to his/her conduct that make the conduct criminal. Beyond this, the mental states referred to in §§ 1512(a), 1512(b), and 1512(c) differ slightly.
Sections 1512(a) and 1512(b) require, in addition to general intent, a specific intent, for example, the intent to influence testimony in an official proceeding. These requirements of specific intent are self-explanatory. In contrast, § 1512(c) does not require specific intent but specific results, for example, preventing a witness from testifying at an official proceeding. However, this distinction is probably without a difference, and the specific results should be read as forms of specific intent. Section 1512(d) codifies existing case law that holds that influencing a witness is not a strict liability offense. See United States v. Johnson, 585 F.2d 119, 128 (5th Cir. 1978). One may influence a witness to tell the truth. See id. However, under 18 U.S.C. § 1512(d), the burden of proving this benign intent, which is an affirmative defense, is on the defendant. A preponderance of the evidence is the standard of proof.
Section 1512(f) of Title 18 contains an important qualification of the mens rea required under the statute: it obviates the need to prove that the defendant was aware of the official nature of the proceedings or investigation with which he/she interfered. A reference to congressional proceedings, however, is omitted from the proceedings enumerated in 18 U.S.C. § 1512(f).
[cited in Criminal Resource Manual 1734; JM 9-69.100]