Sorry, these words have a specific meaning in a legal context.
http://recoverymodel.com/id14.html
> An imminent danger is an anticipated danger that is likely to happen, is impending, and is separated by space or time. This impending danger could be one hour, one day, or an unknown time away but is still imminent.
https://www.osha.gov/as/opa/worker/danger.html
[Imminent Danger is defined here, but Imminent is used in a similar context to Imminent Threat]
> any conditions or practices in any place of employment which are such that a danger exists which could reasonably be expected to cause death or serious physical harm immediately or before the imminence of such danger can be eliminated through the enforcement procedures
http://thelawdictionary.org/imminent-danger/
> In relation to homicide in self-defense, this term means immediate danger, such as must be Instantly met, such as cannot be guarded against It calling for the assistance of others or the protection of the law.
The shooter had a firearm, had used it, and had communicated an intent to use it again in the near-future. If any police officer had walked into the shooters line of fire it would be reasonable to assume that they would be shot. Thus, it is a _textbook_ definition of imminent threat.
I know that the legal context often uses English words in manners that are very similar to English, but do not share the precise definition. Right now, you are mistakenly using the English definitions in a legal context. You need to use the legal definition. And if you put this in front of a judge they will tell you that this is a clear-cut example of an imminent threat.