Is the manner of deaths relevant?
Was the manner of death the same for all of the victims?
(If yes to both of the above) Were the deaths by poison?
At the time that he called off the investigation, would Smith have said that the deaths were murder?
**Detective Smith and his team were aggressively pursuing the tail of the worst serial killer in the history of their police force. 40 murders or attempted murders, probably more that went undiscovered. Suddenly, Smith and his team called off all investigations. Why?
**
NO
kk
So Smith learned new information, but not about the suspect?
Did he call off the investigation on the whole set of murders?
Only on the suspect?
Were the alleged murders actual murders?
Which century? XXI? XX? XIX? XVIII? XVII?
kk
At the time that Smith called off the investigation, was there, to the best of Smith’s knowledge, exactly one murderer?
At the time that Smith called off the investigation, was there, to the best of Smith’s knowledge, more than one murderer?
kk
Does “calling off the investigation” mean they finally had enough evidence to convict?
Was jurisdiction lost because of shifting national boundaries?
Was jurisdiction lost because of a political action?
**scratch previous response; i misread the question!
**
NO to all.
Was there something about the suspect that exculpated him?
Was there something about the victims that exculpated the suspect?
Was there some sort of forensic improvement that exculpated the suspect?
You guys are going down the right paths of thought. This one is kind of fun. 
Was there an assumption made that they realized was not true?
For example, were the bodies of the victims unusually well-preserved, so that they were assumed to be more recently killed?
Was the investigation called off because it would be embarrassing to the government of the jurisdiction?
Was it called off because it was thought that the perpetrator would not be a threat anymore?
Would the perpetrator actually not be a threat anymore?
I forget if this was covered – was the perpetrator actually human?
kk
Was the suspect physically incapable of committing the murders?
Did the suspect have an alibi? Meaning they were proven to be elsewhere during the murders.
Was the suspect in prison?
Was the suspect one of the murder victims?
Had the suspect changed identity?
Had somebody else framed the chief suspect?
Did the suspect make some kind of deal with prosecutors?
kk
Did Smith learn something about the murderer that proved it wasn’t the suspect?
Did Smith learn something about the suspect that proved he wasn’t the murderer?
Would an outside observer (such as the FBI) conclude that Smith should never have pursued the suspect in the first place?
I carefully read and re-read your first two questions to make sure I understood their difference and meaning. I believe I answered correctly. They did NOT learn something about the murderer, but DID learn something about the suspect.
Was the thing they learned about the suspect…
A physical or mental disability or illness of some sort?
A physical trait that is not usually considered a disability? (E.g., blood type, height, left-handedness, etc.)
A skill the suspect did not have? (E.g., the suspect can’t drive, and the murders must have been committed by someone who could?)
A skill the suspect DID have? (I’m not even sure what an example of this would be, but who knows…)
Some aspect of the suspect’s background or past?
There is a real perpetrator out there, correct, even though this particular suspect is innocent?
If yes, is there some (relevant) reason Detective Smith did not continue the investigation until the real perpetrator was caught?
Was there a physical attribute of the suspect that proved he couldn’t have been the murderer?
Gender?
Mental?
Age?
Other?