Do you, the jury, unanimously find that the Government has established beyond a reasonable doubt that the defendant participated in the act, i.e. lying to federal agents on August 16-17, 2001, contemplating that the life of a person would be taken or intending that lethal force would be used in connection with a person, other than one of the participants in the offense.

The Government had argued (pdf) that "failure to tell the truth" is subsumed within lies.

There can be no mistake as to what our theory has been, and there should be no mistake as to what it is now: the defendant's act -- whether called an act of lying, an act of deception, an act of concealment, or an act of withholding the truth -- caused at least one death on September 11, 2001. By whatever name, it is an act -- an affirmative, calculated, deliberate effort to prevent law enforcement from stopping an ongoing plot. And that act includes, for causation purposes, not only what the defendant did say, but the truths that he purposely concealed.

The defense maintains,

[T]he Government's claim that Defendant's lies directly resulted in the deaths of one or more victims on September 11 . . . is not a claim that the Government could have prevented a death if the Defendant had told the truth.").

It's a Fifth Amendment issue. Once the defendant invoked his right to counsel, he no longer had to tell the Government anything about 9/11. He should only be liable for his lies before he decided to cease talking.

It's also an issue of statutory interpretation. The death penalty statute requires there to be an "act." It never mentions "omissions."

18 U.S.C. § 3591(a)(2)©. The FDPA provides that a defendant is eligible for the death penalty if the jury finds, beyond a reasonable doubt, that the
defendant "intentionally participated in an act, contemplating that the life of a person would be taken or intending that lethal force would be used in connection with a person, other than one of the participants in the offense, and the victim died as a direct result of the act[.]"

I think "an act" requires affirmative action, like lying. and cannot be said to encompass failing to take action, like failing to tell the truth.

There's more discussion of this here.