If a firearm or any other object is used to cause the death of a human being... that firearm or device will be taken into custody as evidence. Evidence works two ways... It can tend to convict someone... or tend to exculpate them (proof of being not guilty of a crime).
A criminal investigator will have no legitimate reason to sieze other property including firearms, unless he has a reason to believe they constitute an ongoing threat or are a danger to others, or the well being of the subject. So if an Officer believes that a subject of an investigation is so distrought over his actions that he may do himself harm... He'd have a reason to sieze other firearms. Or if the subject is making statements about revenge, or makes threats to anyone... His weapons may be subject to siezure.
Unless I'm being misled by my friends in the legal profession all siezures are subject to judicial review at some point.