If I were to impart one thing about detecting—from my years as a criminal investigator—it is this: there's always one thing, which proves or disproves a persons guilt or innocence. And, that one thing needs to always be the lens with which the rest of the investigation is examined through.
This is not to infer that an investigator doesn't have to collect every item of evidence, interview every possible witness, and always remain impartial. But, I recall what happened when investigators (and lawyers, and judges) failed to keep the investigation (court proceedings, trial) focused through that one thing; the result was almost always the same: guilty people were not held accountable for their crimes.
Bad people go free when they (or their lawyers) cause law enforcement to lose focus on the one thing.
When I explained this #1 Rule of Thumb to my subordinates, I referred to it as the Bloody Socks Rule. I contend that the OJ Simpson trial would have resulted in a conviction if the prosecution spent a few days showing the jurors what the crime scene looked like and then explained about the socks collected from the floor of OJ's bedroom—which had the blood of OJ, Nicole, and Goldman on them—and then said, "the prosecution rests your honor." Forget about the Bruno Magli shoes, Kato Kaelin, Mark Furman, and the bloody gloves; the one thing is the socks with three peoples blood on them.
Any case can be fogged by the "what about..." and "explain the..." but as long as the one thing is kept in the forefront—bad people are caught and put in prison.
The assassination of President Kennedy is a famous example of an investigation that has been so incredibly inundated in evidence and investigation and re-investigation, that most people believe there was a conspiracy (by some large government organization).
I know there was no conspiracy. Lee Harvey Oswald acted alone. Jack Ruby acted alone. The reason I know is because of two one thing's (one for Oswald and one for Ruby).
Oswald moved (back) to Dallas Texas in October of 1963 and got a job at the Texas Book Depository. Over six weeks later (mid-November) the "parade route" of the President's motorcade was published in the Dallas Newspaper, showing it would pass in front of the Book Depository. Oswald arrived at work the morning of 22 November 1963 with a long item wrapped in newspaper; he told co-workers it was curtain rods. [Although it's a second thing it is helpful to also know about Oswald's mindset: that (according to his wife) he tried to assassinate Retired General Edwin Walker with the same rifle in April 1963. The bullet hit a window frame; fragments injured Walker in the arm.]
This is Oswald's one thing because when he got the job at the Depository, the route was not yet decided by the Secret Service.
Ruby always carried a pistol and was a "cop groupie" (cops drank at his strip club for free; Ruby frequently hung around the police station). The morning of Sunday, 24 November 1963, Ruby got a phone call from one of his employees, asking him to wire her $25. He took his favorite dog, Sheba, and wired the money a little after 11am. Then, leaving Sheba in his car, Ruby walked a block to the police station, arriving about four minutes before Oswald was escorted out.
This is Ruby's one thing. His timing and the presence of Sheba shows that he was acting on impulse.
Most people in this society who aren't actively mad are, at best, reformed or potential lunatics. — Susan Sontag