The terrorist attack was planned in advance. The murder that occurred in the act of that crime thus falls under "felony murder" and probably could be 1st degree murder since the crime that led to the murder was planned. Felony murder is crazy.
But the thing is, felony murder is bullshit. It makes murderers out of people who are not actually killers. Felony murder lets prosecutors charge people with murder if someone dies, even by accident, if they were committing a dangerous felony at the time of the death. The underlying felony has to be dangerous enough that someone dying, even by accident, was foreseeable. A common application of felony murder is when a group of people plan a robbery and someone dies. Even if the death is accidental or one of the perpetrators is killed by a cop. It doesn't matter what role you played in the underlying felony, or even if anyone was armed.
Because the crimes were committed in DC, federal law applies. The federal felony murder rule, found at 18 U.S. Code 1111, says people can be charged with murder if they: "committed in the perpetration of, or attempt to perpetrate, any arson, escape, murder, kidnapping, treason, espionage, sabotage, aggravated sexual abuse or sexual abuse, child abuse, burglary, or robbery; or perpetrated as part of a pattern or practice of assault or torture against a child or children; or perpetrated from a premeditated design unlawfully and maliciously to effect the death of any human being other than him who is killed, is murder in the first degree." Armed insurrection, riots, and terrorism are, for whatever reason, not on the list. That doesn't mean it would be impossible for anyone to be charged with felony murder, though. According to former federal prosecutor Tim Heaphy, "You could look at the storming of the doors of the Capitol by that riotous mob as burglary. If they went in with the intent to steal, disrupt proceedings, to commit other offenses, they committed burglary. And, therefore, a murder that occurred during the course of a burglary could be potentially charged as felony murder."
TL;DR: Felony murder is fucking crazy and should go away. But it exists and is on the books, and is used regularly to fuck over people, and these terrorists committed these acts w/ felony murder on the books, so...