Eventually, Elaine discovered that Mona's old school principal, Mr. Waddington, was the murderer. To cover his tracks, he kidnapped Elaine and tried to murder her, too.
In the end, Elaine figured out that Mr. Waddington, the old school principal of Mona's alma mater, was the perpetrator. In an effort to hide his tracks, he took Elaine hostage and tried to murder her too.