In this weekly column "Cross Exam," Elie Honig, a CNN legal analyst and former federal and state prosecutor, gives his take on the latest legal news. Post your questions below. The views expressed in this commentary are his own. View more opinion on CNN. Watch Honig answer reader questions on "CNN Newsroom with Ana Cabrera" at 5:40 p.m. ET Sundays.

(CNN) During the House Intelligence Committee's investigation of President Donald Trump's effort to pressure Ukraine into announcing an investigation of the Bidens, the committee issued 71 subpoenas and requests for information. The Trump administration, in response, produced nothing -- not a single piece of paper.

So, it's only fair to ask: What are they hiding?

We just got our first small hint, and the answer is alarming. A federal judge ordered the administration to turn over documents subject to a Freedom of Information Act lawsuit filed by the Center for Public Integrity. And just one of those documents -- an e-mail from Michael Duffey , a Trump appointee who served in the Office of Management and Budget, to other OMB and Pentagon officials -- gives us a tantalizing look at the administration's efforts to withhold foreign aid to Ukraine, and to cover up its own conduct.

Duffey -- one of four witnesses requested by Senate Minority Leader Chuck Schumer in Trump's upcoming Senate impeachment trial -- sent the e-mail roughly 90 minutes after Trump's July 25 phone call with Ukrainian President Volodymyr Zelensky. The Duffey e-mail is a prosecutor's dream, opening up various lines of potentially fruitful questioning.

First question: Who gave you the referenced guidance? Second question: What was the guidance? Clearly, somebody in the White House who was senior to Duffey told him to make sure to hold back foreign aid to Ukraine -- and did it right after Trump's call with Zelensky. The answer to these questions could provide information vital to understand Trump's actions and motives toward Ukraine.

Duffey later instructs in the e-mail, "Given the sensitive nature of the request, I appreciate your keeping that information closely held."

Ok, Duffey: explain the "sensitive nature" of the request to hold back funding from Ukraine? Why was it sensitive? Who told you it was sensitive? The answers to these questions could prove damning to Trump and his most senior officials.

Remember: the Duffey e-mail is just one piece of paper out of likely thousands, or more, that the administration has but refuses to turn over. If this one e-mail raises so many important questions, just imagine what we don't yet know.

I believe we will see many or perhaps all of the documents that the administration is withholding -- somehow, someday. The documents might come out, like the Duffey e-mail did, through Freedom of Information Act requests. Or they might become public through other lawsuits, subpoenas by Congress or prosecutors, disclosure by future administrations, or leaks. But, one way or another, we will eventually learn what this administration may be hiding.

The problem, of course, is that we may not know what the Trump administration is covering up until it is too late -- until this administration is long gone, or until it is too late to fix the damage.

Now, your questions

Scott (Tennessee): If the Senate acquits Trump, but then he does something else later in his term that merits impeachment, can the House impeach him again? Or, is there a double jeopardy rule against impeaching a person twice?

No, the double jeopardy rule does not apply to impeachment. Double jeopardy is a concept unique to criminal law. The Fifth Amendment of the Constitution provides that "nor shall any person be subject for the same offense to be twice put in jeopardy of life or limb."

The "in jeopardy of life or limb" language refers to criminal penalties -- including imprisonment or execution -- and not to impeachment, where the only penalty is removal from office.

While the double jeopardy rule means that a person cannot be charged twice for the same criminal offense, the Constitution provides no such legal protection against a second impeachment. No federal official , however, has ever been impeached twice in the United States.

Although second impeachment is legally possible, practical obstacles certainly would arise. It seems exceedingly unlikely as a political matter that the House would impeach a President, see that President acquitted in the Senate and then impeach him again for the same conduct.

If, however, a President was impeached, acquitted, and then committed new impeachable acts, the House could be more willing to impeach again.

Turtley (New York): House Republicans claim that all the evidence against Trump is indirect. Are criminal cases ever decided solely on indirect evidence?

Yes, a jury can convict in a criminal case based solely on indirect (also called "circumstantial") evidence, if it finds that indirect evidence sufficient to establish guilt beyond a reasonable doubt.

Here's how judges explain the concept of direct versus indirect evidence to juries. Let's say you looked out the window of the courtroom and saw raindrops falling down and hitting the window. That would be direct evidence that it is raining outside. Now let's say the window shades were drawn, but you saw three people walk into the courtroom, all wearing raincoats, holding umbrellas and dripping wet. That would be indirect -- but still relevant and potentially decisive -- evidence that it is raining outside.

House Republicans' claim that the case against Trump consists of "no direct evidence" is not true, in any event. The July 25 call between Trump and Zelensky, for example, is direct evidence: you can look at it and see Trump's own words to Zelensky, much as you could look out the courtroom window and see raindrops.

There also is plenty of indirect evidence. For example, Ambassador Bill Taylor testified that it was unusual and unexplained that military aid to Ukraine was held back. A reasonable juror could -- but need not necessarily -- infer from this indirect evidence, all with the other indirect evidence, that the aid was held back in order to pressure Ukraine.

Marty (Texas): Now that Trump has been impeached, does he lose the ability to issue pardons?

No, but the confusion here is understandable. Article II of the Constitution provides that the President "shall have power to grant reprieves and pardons for offenses against the United States, except in cases of impeachment."

On this point, the Constitution -- venerable document that it is -- loses points for ambiguity. Read one way, it could appear to say that after the President is impeached by the House, he loses the power to issue pardons. But read another way, it says that the President can issue pardons for criminal offenses but not for impeachment.

The latter reading is correct . There is no provision anywhere in the Constitution, statutes or case law that strips a president of any power upon impeachment by the House (though of course, if convicted in the Senate, the President loses office and all of its powers). It would be anomalous for the President to lose only one power -- the power to pardon -- upon impeachment alone, and no serious legal authority has argued for this interpretation.

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Rather, the clause in Article II means that while a President can pardon an official (or any person) for a crime, he cannot pardon an official out of impeachment. In other words, the President does not have power to un-impeach. For example, if a federal judge committed bribery, the President could pardon the judge from a criminal bribery charge, but the President could not rescue the judge from impeachment. Indeed, no president has ever pardoned or even attempted to pardon an official from impeachment.

This makes sense in our broader system of checks and balances. The Constitution grants Congress the "sole power" to impeach, as one of its most powerful checks on the President and the executive branch. If the President could simply issue pardons to undo an impeachment, he could unilaterally deprive Congress of one of its vital tools, effectively rendering it moot.

Three questions to watch

1) How long will House Speaker Nancy Pelosi wait before formally sending Articles of Impeachment to the Senate?

2) Will Senate Majority Leader Mitch McConnell and Minority Leader Chuck Schumer make any progress toward a negotiated deal for Trump's Senate impeachment trial?

3) Will Trump and McConnell pursue an aggressive trial strategy, or will they go with a minimalist approach to get it over with quickly?