The police is obliged to remain neutral and present all evidence no matter if it's in- or exculpatory. It up to the lawyers to choose where to put an emphasis on and up to the court to judge which one is relevant and which not.
It was pity stayed his hand. "Pity I don't have any more bullets," thought Frito. -- _Bored_of_the_Rings_, a Harvard Lampoon parody of Tolkein