There have been serial killers throughout US history.
Here is the Court TV crime library on serial killers throughout history, but with an emphasis on the US. Reporters tend to think because it's the first time they've heard of something, it's the first time it's happened. The "end of innocence" line is used constantly as if specific killings were some kind of turning point. They used it with the Boston Strangler, and with Truman Capote's
In Cold Blood, about the murder of the Clutter family. I don't know if there are more serial murderers now. John Wesley Hardin supposedly killed over 30 men.
Richard Speck and the Boston Strangler were probably more along the lines of being the first sensationalized through the media as serial killers.