This scenario played out in Florida recently when two detectives showed up at a funeral home to find a dead man's finger prints. The police had the phone, but couldn't unlock it. Prior to the man's death, there was an incident involving the police, the man and at least his car. The man tried to drive away prior to the police conducting their search of his person. However, his attempt to flee somehow resulted in his death when he was killed by the Largo police. Now, the police were investigating the incident and maybe other events and needed to unlock his phone. They arrived at the funeral home and held his hands up to the phone and used his fingers to see if any of them would unlock the phone using the fingerprint reader. It didn't work.
The sad reality is that this is actually legal because a dead person cannot assert their fourth amendment protections against illegal search and seizures. One does wonder though, even if it is technically legal to do, is it right? Should the police be able to use your loved one's dead body to try to further their own investigation by unlocking his private cell phone?