I have been thinking about this, and it makes sense and is possible, maybe. The killer is a male, per DNA and msm, I will have to get link though. But if there is no match in the dstabase, yet they suspect someone, I am sure they have collected his DNA. ( maybe on the sly or with probable cause, fingerprint in house, being in house for a known reason, etc).And since there is no arrest, it may be just as you say orangecrush, they don't have enough to convict because his DNA may have a logical reason to be there, or at least could be argued away in court, therefore not getting a conviction. The boyfriend has been ruled out, link below.
http://www.cbs5az.com/Clip/12218955/vigil-planned-for-monterey-murder-victim-allison-feldman
But the killer could have been there at some point previously ...and the dna evidence is not enough to convict. I just read a case where a man was charged in Shrewsbury mass.for a killing that took place 40 years ago because his dna was found at the scene and just analyzed. It was a brutal killing, and his drop of blood was near the door. He was recently aquitted!! He delivered a hutch to her the day before. Dna explained away. So....this could be SPD challenge.
And I am speculating only, but maybe the dna was not good under her nails because of the bleach.
Added/ edit- the aquitted man was arrested for beating up and sexually assaulting a woman two days after the Shrewsbury murder, near by, but it wasn't allowed in as evidence, the jury never knew. So they bought the hutch cutting his hand theory.