An American does not have to speak with a government agent, unless the he/she has been arrested. An American has a right to privacy to be left alone.
The PRIVACY ACT OF 1974 (Public Law 93-579), empowers citizens to require full, written disclosure from a government official who seeks information. You may insist on complete disclosure as a precondition to speaking with any government official.

Law-abiding citizens are sometimes visited by agents of the Federal government for no apparent reason. It is helpful, at the time of these visits, to recall that unless a citizen has been placed under arrest (either because a law enforcement officer has probable cause to believe the citizen has committed a crime or because the officer has in his possession an arrest warrant issued by a judge, who believes there is probable cause the citizen has committed a crime, a citizen does not have to entertain the company of government agents.
Citizens also have the right, guaranteed by the Fifth Amendment to the United States Constitution, not to testify against themselves. Thus, when "the government" comes knocking on one's door, you have the right to simply say, "Please go away." Unless the government officer places you under arrest (there must be probable cause, or an arrest warrant based on probable cause), the officer must obey your wishes.

Of course, citizens also have a vested interest in assisting "the government" in its role of crime-solver. Most of us understand the need to help "the government" to apprehend criminals. But it is also helpful, when "the government" arrives at your place of employment or at your home, to know how to find out why government agents have appeared on YOUR doorstep.
A handy little questionnaire that I came across years ago will do the trick. It's called the "Public Servant Questionnaire." A version accompanies this article. The "PSQ" was developed by Lynn Johnston, author of Who's Afraid of the IRS? (Libertarian Review Foundation: 1983, ISBN 0-930073-03-7).
The PSQ is based on the requirements placed upon the government by the Privacy Act of 1974 (Public Law 93-579), an amending law to Title 5, United States Code, Section 552, and is included as Section 552a. If a citizen chooses to cooperate with government officials who are seeking information, BEFORE questioning begins, the citizen should politely inform the government agent or agents that a prerequisite for the citizen's cooperation with "the government" is the agent's cooperation with the citizen.

The questions should then be put to each agent, and the citizen should enter the answers onto the questionnaire. Copies should be provided to each agent, either at the time of the questioning or by mail to the agent after the visit. The questionnaire informs the government agent that the citizen knows his rights and knows which limited powers the government agent has been granted by the people.
Most probably some government agents will not want to fill out or sign the PSQ. That's fine. They can then be sent on their merry way. They may need to explain to their superiors, and a court of law, and a jury, on another day, why they refused to cooperate with the reasonable questions of the highest officeholder in the land, a citizen.

1,2,3 & 4 In order to be sure you know EXACTLY to whom you are giving Information.
5 ALL public servants have taken a sworn oath to uphold and defend the U.S. Constitution.
6 & 7 This is standard procedure by government agents and officers.
8,9 & 10 Title 5 USC 552a, paragraph (e) (3) (A)
11 Title 5 USC 552a, paragraph (d) (5), (e) (1)
12 & 13 Title 5 USC 552a, paragraph (e) (3) (B), (e) (3) (C)
14 Title 5 USC 552a, paragraph (e) (3) (D)
15 Public Law 93-579 (b) (1)
16 Title 5 USC 552a, paragraph (e) (3) (A)
17 & 18 Title 5 USC 552a, paragraph (e) (2)
18 Title 5 USC 552a, paragraph (d) (5)
20 & 21 Public Law 93-579 (b) (1)
22 Title 5 USC 552a, paragraph (d) (1)
23 Title 5 USC 552a, paragraph (e) (10)