This would be a serious crime
Knowingly telling lies to a federal investigator is itself a crime, or can be. Threatening someone to attempt to indue that person not to cooperate with a federal investigation is also a crime. The shotgun casing would be likely to be considered a threat, and later efforts to pass it off as a "joke" are not likely to work. Thus at least two crimes would be involved, quite possibly more.
False Statement 18 UDC 1001
Such a person would be violating 18 USC 1001(a) which provides:
(a) Except as otherwise provided in this section, whoever, in any matter within the jurisdiction of the executive, legislative, or judicial branch of the Government of the United States, knowingly and willfully—
(a)(1) falsifies, conceals, or covers up by any trick, scheme, or device a material fact;
(a)(2) makes any materially false, fictitious, or fraudulent statement or representation; or
(a)(3) makes or uses any false writing or document knowing the same to contain any materially false, fictitious, or fraudulent statement or entry;
shall be fined under this title, imprisoned not more than 5 years or, if the offense involves international or domestic terrorism (as defined in section 2331), imprisoned not more than 8 years, or both. If the matter relates to an offense under chapter 109A, 109B, 110, or 117, or section 1591, then the term of imprisonment imposed under this section shall be not more than 8 years.
See also "What are “false statements” under 18 USC Section 1001" which states
False statements can be spoken or written and do not have to be made under oath to violate the law, applying equally to misrepresenting income to the IRS or lying to the FBI during an interview. The government can’t convict a person simply for telling a lie. In addition to proving that the defendant made the statement in question to a federal agent or officer, the government must also prove three things:
- That the defendant’s statement was “materially” false. ...
- That the defendant “knowingly and willingly” made the false statement. ...
- That the statement was made regarding a matter within the federal government’s jurisdiction. ...
Tampering with a witness 18 USC § 1512
Such a person would also be in violation of
18 USC § 1512 - Tampering with a witness, victim, or an informant subsection (a)(2) which provides that:
(a)(2) Whoever uses physical force or the threat of physical force against any person, or attempts to do so, with intent to—
(a)(2)(A) influence, delay, or prevent the testimony of any person in an official proceeding;
(a)(2)(B) cause or induce any person to—
(a)(2)(B)(i) withhold testimony, or withhold a record, document, or other object, from an official proceeding;
(a)(2)(B)(ii) alter, destroy, mutilate, or conceal an object with intent to impair the integrity or availability of the object for use in an official proceeding;
(a)(2)(B)(iii) evade legal process summoning that person to appear as a witness, or to produce a record, document, or other object, in an official proceeding; or
(a)(2)(B)(iv) be absent from an official proceeding to which that person has been summoned by legal process; or
(a)(2)(C) hinder, delay, or prevent the communication to a law enforcement officer or judge of the United States of information relating to the commission or possible commission of a Federal offense or a violation of conditions of probation, supervised release, parole, or release pending judicial proceedings;
shall be punished as provided in paragraph (3).
18 USC § 1512 (a)(2) carries a penalty of up to 20 years imprisonment, plus a fine. 18 USC § 1512 (d) could also be charged here, it carries up to 3 years, plus a fine.
There might also be charges for harassment or stalking, depending on state or local law. There could also be a charge of accessory to or conspiracy with the person being investigated.
In short, such a person could get into some quite serious trouble, if the investigators found out what s/he has done.