Is there an official document showing the SPD made such a finding?
Serino filed an affidavit saying there was sufficient evidence to prosecute, without addressing the justification claim.
Serino had to invent a hitherto unknown legal standard, to wit, "Zimmerman could have prevented the outcome" (by staying in his vehicle or by apprising Martin of his concern), in order to justify the conclusion that the state attorney could/should charge Zimmerman. If the standard for prosecution is "Zimmerman got out of his truck," then yes, there is plenty of evidence. But that is NOT the standard, and I have not seen any document that sets for the evidence vs. the legal standard, that justifies charging Zimmerman.
It isn't an official document, per se, but Chief Lee explained, and the explanation was published to the community, that the police lacked probable cause to arrest, and the city did not arrest. The only difference between the state's evidence and the SPD's evidence is Witness 8, and Sybrina Fulton.