In theory, you could sue him and the police department for having negligently or carelessly lost your property; it is better to name both him and the department to be thorough, though the officer himself will likely have substantial protection from lawsuits coming out of his official business, as long as he was not acting in an intentionally wrongful manner. In practice, it's not clear this would be worthwhile:1) There are extra paperwork requirements and rules about suing the police (or any government agency) and shorter deadlines to file--therefore either you'll have to hire an attorney, whose cost you cannot be reimbursed for, to help you, or else navigate a more-complex suit yourself.2) You can potentially recover the monetary value of the knife, assuming you can prove a) that the officer caused its loss and b) its current fair-market value, but not the personal value...so this would only make sense if the monetary value alone is worth going through 1), above.