They like to have things pretty well cut and dried before they act.

A prosecution is an expensive affair, so they don't care to prosecute unless they are fairly sure of a conviction.

If they fail they get hauled over the coals." "But don't you think they would get a conviction in this case ?" "Not on your evidence, Jervis.

They might pick up something fresh, but, if they didn't they would fail.

You haven't got enough hard-baked facts to upset a capable defence.