Angela Brandt's fingerprints were found on the murder weapon, although she didn't commit the murder. She couldn't have tampered with the evidence because (1) she didn't have time, as Julian Decker's body was found very shortly after he was killed; and (2) she couldn't have been protecting the real killer because, the first time the police talked to her, Angela was just learning about the murder and didn't know who killed Decker.
The DNA sample obtained from the underwear were supposedly an 80 percent match to Angela. Since it turned out that it was actually her daughter's DNA it would be scientifically impossible to be more than a 50 percent match to Angela since a person gets 50 percent of their DNA from each parent.