It's difficult to tell. The paperwork in the 70s wasn't as well-organized or documented as it is today, but I'd wager a guess and say they didn't realize it until 1973 when the FBI was called in.
Local law enforcement handles a large number of murders and related disturbances every day, so it can be difficult to link two or several murders together. The FBI can't get involved without an invitation, so even though we're structured to handle a smaller amount of special cases we can't really spot a serial killer before the local police does.