A month ago, police investigating the murder of Wenatchee teen Mackenzie Cowell said they were “cautiously optimistic” about some new tips. The 17-year-old high school senior was found stabbed and strangled on the banks of the Columbia River in February. Despite hundreds of interviews and even more leads, investigators have yet to name a suspect. But they do think they may found an important clue.Today police released pictures of a particular type of knife recovered near where Cowell’s body was found: a Maxam brand blade with a unique black-and-gray handle.Cops aren’t saying whether the knife itself was used in Cowell’s murder. And they’re not giving any hints as to whether or not they pulled a usable print off of it.But they do seem to think that the knife represents a trail they can pursue. Only one store in Wenatchee — Stan’s Merry Mart — has sold the set within the past two years, and two others sets were given away at a nearby restaurant as a door prize.