They said the DNA they collected was a transfer DNA, which means one of the suspects must have had physical contact with the officer in one form or another (or something at the scene).
What kind of DNA could be collected ? Could've been sweat, spit, blood, skin cells, urine.
What kind of transfer may have taken place ? Officers gun had sweat DNA on it, skin cells under officers fingernails, blood left at the scene by perp, sweat DNA on officers skin or clothing, cigarette butts at crime scene, coffee cups or drinking cups at crime scene, urine found on surrounding ground or foliage.
It's too bad the videos came up empty. The DNA is a powerful tool though if they can find a perp to match it to.
They have no DNA match, meaning the person who left the samples has never been tested, which rules out most convicted felons.
Of course, probably upwards of 90% of the general public has never had DNA extracted or offered samples.
They still have a lot of work left to do. This case may go cold if someone doesn't step forward either for the reward money or because they have a good idea who is behind it.