I am not so sure you can't do a rapid and thorough investigation. It depends upon the complexity of the case and how many people you can put on it. In this case I don't see things taking all that long. 1 man goes out and murders 16 people then walks home. The details of the crime itself won't be that hard to pin down.

From the standpoint of a defense lawyer and a prosecutor it may take longer to count as thorough because motivation comes into play (this is a guess Mike, please correct if needed). They may be looking forward to the trial, a long time in the future. But I think if an investigator is looking at why, just ask the guy today.

Sometimes I think Americans equate length of an investigation with thoroughness because in our country, there are so many games within games involved.