In order for a search warrant to be issued the police must have more than a mere suspicion a crime has happened. They must good reason to believe that a crime has happened and then have probable cause (as much as that's an American term it's accurate) to believe the place they want to search has evidence relevant to that crime.
The police then put their information in a document called an Information to Obtain (ITO). The ITO can be enormous – it can amount to several bound volumes if the matter is very complex. In the ITO the police outline the information they have – and they must be complete and include everything, even the information that may be contradictory to their case. Nothing can be hidden as the Justice of the Peace who will decide whether to grant the warrant must be fully informed. If something was suppressed or not disclosed the warrant may be invalid and evidence found as a result of searches made pursuant to the warrant may be inadmissible at trial.
A very important point is that because the police must disclose everything, a lot of material in an ITO is not the sort of evidence you would hear at a trial. Whatever the police know they must disclose -- that means that what is in an ITO often contains much hearsay, opinion and other items that show the state of the investigation but are not the sort of thing proper to use to prove someone is actually guilty of a crime.