A suspect is in custody, police said.
Prosecutors are poised to request that a grand jury indict Elias Rodriguez as soon as this week, sources told ABC News.
Boulder County District Attorney's Office said in a statement Monday that Karen Diamond died as a result of "the severe injuries that she suffered in the attack."
Mohamed Soliman, 45, appeared in federal court in Denver for his arraignment after being indicted on a dozen federal charges in connection with the June 1 attack.
A man who told investigators he was driven by a desire "to kill all Zionist people" when he threw Molotov cocktails at demonstrators raising awareness of Israeli hostages appeared briefly in federal court for the first time Friday to face a hate crime charge.
According to state police, the two chased another employee around the firehouse with a rope, shaped like a noose.