In the presentation of evidence against the suspect, they state the killer used the amputated fingers of the victims to operate his phone. The phone shown in the evidence looks to be a late model smart phone, in line with the technologically savvy victim. The fingerprint ID for the phone would therefore need a live finger to operate as the technology relies on electrical conductance as opposed to older phones that use optical sensors.
Roz states that Tim Ifield had a list of passwords in the Notes app on his phone. This is highly irregular given what is known about Tim's obsessiveness as regards security and his detailed knowledge of forensics - it would be uncharacteristically reckless of him to have passwords in such a poorly secured location on his phone.