A southwestern Illinois trial is underway for a man accused in a decades-old killing of a teenager.  The Belleville News-Democrat reports testimony began Tuesday in the proceedings involving 52-year old Carlos Garrett.  A prosecutor told a Saint Clair County jury that genetic evidence links Garrett to the 1989 death of 16-year old Cahokia High School junior Nicole Willis.  Her half-naked, beaten and strangled body was found in a field near her home.  Garrett was charged with the murder after an Illinois State Police investigator reopened the case last year while Garrett was serving prison time for a drug conviction.  Authorities say DNA evidence taken from under Willis’ fingernails was matched to Garrett.  Garrett’s attorney has told jurors not to trust the science, and that evidence was mishandled by police.