The methodology for obtaining that confirmation was standard in structure, though its application to a case of this significance was unusual in practice: investigators would obtain a biological sample from DeAngelo without his knowledge or cooperation, develop a DNA profile from that sample, and compare it against the crime-scene profile that the entire investigation had been built around.
The legal framework for this kind of covert DNA collection had been established in California and other jurisdictions through case law and statutory interpretation. When someone leaves biological material in a public place or discards biological material in a way that abandons any privacy interest in it — spitting on the sidewalk, leaving a used coffee cup on a table, discarding a tissue in a public trash can — investigators have generally been permitted to collect and test that material without a warrant, because the person who discarded it has no reasonable expectation of privacy in it once it is abandoned.
Investigators placed DeAngelo under surveillance. They waited for an opportunity to collect material he had discarded in a public setting.
The accounts of what was collected vary in their specificity. Published reporting from the arrest period identifies the collected material variously as a DNA sample from a pizza restaurant and from a Department of Motor Vehicles office — accounts that are not necessarily contradictory if multiple collections were made. The identification published by the Sacramento County Sheriff’s Department following the arrest described the DNA as having been collected from material DeAngelo discarded.
The sample was submitted to a laboratory for analysis. A DNA profile was developed.