How to pronounce the "tt" in flatter in american english? "t" sound or "d" sound?

somebody told me "tt" in "letter" and "better" should be pronounced as "d" sound in american english. because it's in the middle of vowels. so i think the "tt" in flatter should be pronounced as "d" sound, but actually i did heard some amercians pronounced it as "t" sound. why? this kind of thing also happened to "forgetten" and "written". i'm totally confused.