Since swords weren't the murder weapon, at least that clears Mai.
It's a localization.
And here we have a lineup of potential suspects
I can't identify the final kanji, but the rest of the phrase written there is:
犯人は乳 (Hannin wa nyuu), which literally means "The culprit has breast-"
It's not the kanji for cow though, and since Vivid subs generally took liberties with the translation (e.g. "itch the ditch") we can be pretty confident it's a localization.
More accurately, it refers to being able to tell the shape of breasts even though they're covered in clothes, which would require there to be "bag" for the breasts sewed into the clothes. Of course, that would only work with big breasts.
there are no cows in amagi brilliant park, silly.
>You'll never have a PJ party with Sento, Muse, and Latifa