DNA samples taken from a murder scene at a Yale graduate scientific laboratory in 2009, revealed two profiles. One matched a co-worker later implicated in the crime through other evidence, and the other matched a convicted offender living nearby. The only problem: The latter had died two years prior to the Yale attack.
It turned out that the offender, who was a construction worker, had built the very mechanical chase in which the victim was found, leaving behind his DNA as he sweated during the construction. His cells were preserved and undisturbed until they were transferred to the victim as she fell through the cramped space.
This phenomenon is called secondary or indirect DNA transfer and means that DNA from one person is transferred to another person via an intermediary person or object, thus without the two people coming in contact with each other.
With todays means to link perpetrators to an offense by the presence of their DNA at the crime scene, it is of utmost important to understand how easy cells can appear in a sample with which the DNA donor has never come into contact. It has to be kept in mind that what looks like a primary DNA transfer might in fact be a secondary transfer.
In the Yale murder case, had he not priorly died, the construction worker might have been implicated in the crime due to his prior record and familiarity with the space.
Source: New Republic