Police use of Time Ships is reserved for the most serious offences; murder, treason, and the like. Since 1895 Scotland Yard has had a Temporal Crimes Bureau, set up to help police from the future. Typically a small group of officers land discreetly, identify themselves at the bureau, then make their way to the crime scene with "local" help. They lay in wait for the criminal, interrupt the crime, and identify the criminal. If possible they will prevent the crime; even though changes in the events of the past don't change the present, it's not in the nature of most officers to let someone get away with a serious crime. All of the evidence including statements from the victim etc. are then taken back to the present. Note that this alone is not sufficient to obtain a conviction, since the defence can (and has) argued that since changes in the past do not affect the present, it's possible that what was observed in the past was somehow different from the original sequence of events. However, the evidence gathered by this technique can be used to obtain search warrants, and to justify forensic work, autopsies, exhumations etc. in the present.
On a few occasions this technique has failed; criminals aware that they might be caught by these means have prepared elaborate deceptions in which someone else appears to commit the crime, or obscured the date and location of the crime so thoroughly that they could not be traced. Fortunately few criminals are bright enough for this. In one case the victim actually lied in her statement to protect the murderer, despite seeing photographs of her own body, but fortunately said just enough to allow the police to find more evidence in the present, and thus solve the case.
The legal implications of this method are a minefield. One defense - that a crime has not been convicted because it was prevented in the past - has already been eliminated by the Law Lords, in a landmark case which established that events as perceived in the present must be the only criterion, with any tampering in the past irrelevant. On two occasions the defence has bought the victim to the present as 'proof' that the crime did not occur; in both cases a conviction was still obtained, since the prosecution could prove conclusively that the murder had taken place.
|Scenario Idea: Ripping Yarns|
Even though they pre-date the Temporal Crimes Bureau the Jack the Ripper murders of 1888 are an obvious target for investigation by this method, but the police seem to be curiously reluctant to make the necessary moves. As a result of recent questions in Parliament there is a general feeling that something needs to be done. Why are the police dragging their heels? Could there be any truth in the rumours of Royal or Masonic involvement?
As the debate continues and the police excuses get less believable a gentleman sportsman proposes to take a hunting party back, entirely unofficially, and get the evidence needed to solve the case. Are the adventurers interested?
There's one tiny snag... the Ripper is aboard the ship, and plans to make sure that the hunt will be unsuccessful. To play fair, he (or she) should be someone known to the adventurers, either as another member of the hunting party or as a member of the crew, criminologist along to study the case, etc.
For more on the Ripper murders see numerous books, web sites, etc., e.g.
A web site outlining the facts that doesn't attempt to draw conclusions.
The official police site on the case. The Wikipedia page on the case.