According to german law, you do not need a transit visa, if you stay inside of the international terminal. That means your luggage must be forwarded and you mustn't leave the terminal or cross the Schengen Area in any form. Was the second leg operated by TK? If not, it is very likely that you'd had to proceed to baggage claim and than check in again at the counter of the next carrier, for which a US VISA only is not enough. In that case you would need a Schengen Visa, sorry.