Renato Mariotti is a former federal prosecutor and a CNN legal analyst.

The decision by President Trump’s former attorney Michael Cohen to postpone congressional testimony because of “ongoing threats against his family from President Trump and Mr. [Rudolph] Giuliani” put the spotlight on highly improper comments by Trump. But it is more likely a tactic by Cohen to avoid being forced to testify on certain topics. And the move will almost certainly prove unsuccessful, merely buying him some time before he ultimately does appear before lawmakers. The Senate, in fact, subpoenaed him on Thursday.

It’s unclear what threats Cohen was referring to, but Trump has tweeted that Cohen agreed to cooperate against him to get “his wife and father-in-law (who has the money?) off Scott Free.” Later, Trump told Fox News Channel host Jeanine Pirro that “people want to look at” Cohen’s “father-in law.” After Trump again tweeted to “watch father-in-law!” Giuliani told CNN that Cohen’s father-in-law “may have ties to something called organized crime.”

As a former federal prosecutor, I find it reprehensible that Trump and Giuliani would try to intimidate Cohen, a witness against Trump, by suggesting that Cohen’s father-in-law should be investigated. Trump has the power to initiate investigations as head of the executive branch, and as president he obviously has immense influence and powerful allies. Trump’s congressional allies investigated former FBI director James B. Comey, another witness against Trump, last year at the president’s urging.

Commentators and lawmakers are correct to point out that this conduct looks like witness tampering. Why would the president of the United States take the time to publicly call out Cohen’s father-in-law if Cohen wasn’t a witness against him? But unless Trump or Giuliani made more direct threats in private, it is unclear whether this would be sufficient to prove witness tampering in court beyond a reasonable doubt.

Congress can still subpoena Cohen to testify, as lawmakers already have vowed to do, as they have the power to obtain his testimony even after he is in custody. But can they compel him to answer questions he doesn’t want to answer? There is good reason to believe that Cohen is holding something back.

Although Cohen has provided helpful information to special counsel Robert S. Mueller III and other federal prosecutors, he “does not have a cooperation agreement” with the U.S. attorney’s office for the Southern District of New York because he “repeatedly declined to provide full information about the scope of any additional criminal conduct in which he may have engaged or had knowledge.”

As Cohen negotiated a plea agreement with federal prosecutors, he had every incentive to obtain a full cooperation agreement to reduce his sentence. But he apparently refused to do so because he was unwilling to come clean about additional criminal conduct of some sort, even though he pleaded guilty to multiple federal crimes in two cases.

That helps explain why Cohen doesn’t want to testify before Congress. If he knows about criminal conduct committed by someone else, he can’t use the Fifth Amendment to refuse to discuss it unless he was implicated in those crimes himself. It’s possible that Cohen wants to protect a friend or associate but can’t do that without risking a separate prosecution for contempt of Congress, which could eventually result in more prison time.

What if Cohen wants to avoid testifying about his own criminal conduct? He can’t shirk testimony about most of the crimes he was convicted of, such as the campaign finance crimes that federal prosecutors allege were directed by Trump, or lying to Congress about the timing of the Trump Tower Moscow talks. That conduct is only a crime under federal law, and Cohen was already convicted of those crimes, which means the Fifth Amendment protections against self-incrimination don’t apply.

Some of what Cohen was convicted of, such as making false statements to financial institutions, could constitute separate crimes under state law. It would be appropriate for Cohen to assert the Fifth Amendment about those actions unless he received immunity from Congress — which lawmakers may be willing to give, because Cohen already pleaded guilty and was sentenced for the same conduct in federal court.

But as to other federal or state crimes for which Cohen still has criminal liability, he can assert the Fifth Amendment, and it’s doubtful that lawmakers would immunize him unless they knew in advance what he would say. If Cohen were to take the Fifth during congressional testimony, the question on everyone’s minds would be: What does he still have to hide? He would then be forced to come clean — and potentially face additional consequences — or be dogged by this question for the rest of his life.

So although Trump’s attempts to harass and intimidate Cohen should be universally condemned, Cohen’s delay really says more about his own conduct than about his former client’s. This is merely his latest attempt to avoid revealing something that he has hidden from federal prosecutors for months. Lawmakers may not be able to force Cohen to spill the beans, but they can shine a spotlight on his refusal to do so.