Using a private investigator can make or break a very close custody case, even if it is just performing an in depth background check, for example in another state where the spouse lived or a comprehensive one in Washington, can make or break a custody case.  I did background checks in Washington state myself for years down at the Courthouse, but did not realize I was missing as much as I was.  The people that opened my eyes to this were a team in south king county whom I highly recommend: www.FRinvestigations.com.  They have substantial experience dealing with custody matters in particular, and if you truly care about your case they can be a good place to start.