From my research into her case a few years ago, it appears her killer was never identified, nor were any strong suspects.
Heidi was 4 when she disappeared from the front yard of her Capitol Hill home near Seattle in February of '74. She and her younger brother were playing in front of the house while her mother went to a local store. When her father went outside some 15 minutes later, her brother, sitting on his bike, simply said "Heidi go".
11 months later, her remains were found in a garbage can near a scenic overlook, only a block from her home. Searchers were dumbfounded, as the area had been thoroughly searched many times in the weeks following her disappearance.