First, let's assemble the parameters necessary to create the impossible crime.
1. At some point in time, between 8 PM and 9 PM on October 14th, a gunshot was heard from Room 913 of Powers Employment Offices.
2. At this same point in time, the bullet that produced the gunshot punctured Groud's left lung, killing him within three minutes.
3. The killer was in the room where the crime occurred.
4. Only Yalt and Groud were at the place of death and the time of death.
5. Groud did not commit suicide.
Examining these, it would be a cheap end if it was a suicide all along. Assuming conditions one and two, condition four is a fact unless a person was able to hide on Floor 9 and then escape the lockdown without suspicion. Condition three is a possible problem, but because Yalt's prints are on the gloves, this is highly unlikely. Condition one is a fact. So, what's left?
The core of my theory is breaking condition two. I maintain that Groud was already dead before the gunshot was heard. The obvious problem is that the gunshot is the listed cause of death in the autopsy report, but the autopsy report is unreliable. In ignoring all injuries that could reasonably have been inflicted by the fall, it prematurely eliminates several potential causes of death. As proof of my theory, I present the Visitor's List and the Cell Phone. Groud arrived approximately two hours before he supposedly called Christine. Either he waited two hours before deciding to do something, or the person who placed the call was not him. Possiblity one is unlikely, so possibility two is favored. If a different person placed the call, they ran the risk of the real E showing up. This leads to a high probability of E being dead before the call was placed.
I accuse Masterson of this murder. Although no solid evidence of his guilt exists, Masterson knew Christine Yalt's name at the time of arrest, even though it was established that the only items she had on her person were the card, the gun, and the cell phone. Further, the killer knew what was happening in court during the time that Masterson was on the stand.
I suspect that Groud escaped through some unknown, coincidental method. Masterson received the anonymous tip about E, but he followed it far before he claims. He found and killed Groud, perhaps through a beating or torque applied to the neck. Perhaps he wanted revenge on Groud for something related to Cross's father. However, Masterson had to conceal the brutality used. He left and summoned Yalt, using his own burn phone. (Masterson used a different alias for entering and leaving.) Yalt knew Masterson and wanted to conceal his crime for some unknown reason. It is likely that she is in law enforcement despite her young age, explaining the presence of gun and bullets as well as the encryption. She willingly took the fall.
She propped Groud's body, somehow unnoticed, against the window and fired. The killing blow was on the chest, so she taped the card to Groud's body over the wound, intent on removing the anomaly with the gunshot. As expected, the force of the bullet shattered the window. Not only did this create a fake cause of death, but the real cause of death was made inconspicuous.