I'm Lucy, and I'd be happy to answer your questions today. I'm sorry to hear that this happened.
There unfortunately isn't any legal right to make police pursue a case, and they can use their discretion to choose not to get a warrant to search the house. If you can get any evidence (like if you have a mutual friend who saw your phone there or if he admitted to taking it), then you can ask them again to prosecute. Another option is to talk to the district attorney who is handling the assault cases and see if they will add claims for the stolen phone.
You can sue your ex-boyfriend for the value of the phone, the cost of having to the case in the first place, and punitive damages for taking it in the first place. The civil equivalent of theft is conversion. The standard of proof in civil court is much lower than in a criminal case. In civil court, the standard of proof is by a preponderance of the evidence. That means, more likely than not, things happened the way you say they did. If beyond a reasonable doubt is proof to a 99.9999% certainty, by a preponderance of the evidence is 50.1%. Your testimony that he took the phone is proof that it happened (and the judge can consider that it wouldn't make sense for you to sue if he'd given it back to you).
It's also possible to file a civil case for each count of assault. Because of the difference in standards of proof, if he pleads guilty or is convicted in the criminal case, you will AUTOMATICALLY win the civil case. All you need to win is to bring a copy of the judgment from his criminal case. But you don't have to wait to file the lawsuit if you don't want to.
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