conventional banks

Is it halal to purchase shares of conventional banks? I have read many articles and islamic decisions on this but there are cloudy directions as to the consideration of the  percentage of the halal or usury earnings of the company, their percentage of forbidden assets, and thier involvement of in other businesses and also thier percentage of interest based deposits and so on. Please give me a concise and direct fatwa on this issue of buying shares of conventional banks. I really need an answer. Jazakallahu khairan.