So why was evidence blocked? Prosecutors wanted to argue Hassan carried out a copy cat attack, and the judge thought that might make things in the court room too confusing.
A military judge blocked several key pieces of evidence Monday that prosecutors said would explain the mindset of the soldier accused in the 2009 shooting rampage at Fort Hood, including his belief that he had a "jihad duty" to carry out the attack.
Prosecutors had asked the judge to approve several witnesses and various evidence to support what they allege motivated Maj. Nidal Hasan to carry out the attack, which killed 13 people and wounded more than 30 others at the Texas military base.
But the judge, Col. Tara Osborn, blocked nearly all of it.
Osborn barred any reference Hasan Akbar, a Muslim soldier sentenced to death for attacking fellow soldiers in Kuwait during the 2003 invasion of Iraq. Prosecutors wanted to suggest that Hasan, an American-born Muslim, carried out a "copycat" attack.
But the judge said introducing such material would "only open the door to a mini-trial" of Akbar and result in a "confusion of issues, unfair prejudice, waste of time and undue delay."
The judge said prosecutors also couldn't introduce three emails, ruling that the needed redactions would make them irrelevant. The contents of the emails weren't disclosed, but the FBI has said Hasan sent numerous emails starting in December 2008 to Anwar al-Awlaki, a radical U.S.-born Islamic cleric killed by a drone strike in Yemen in 2011.