I should know this. I have studied Spanish for years to the point where pret vs. imperf comes naturally to me. However, any information that I find states that saying someone was responsible for something translates as: él/ ella/ Ud. era responsable de...
Why wouldn't it be "fue responsable de ___"? This just doesn't make sense to me. Maybe the logic is that we are unsure of the timeframe for which the subject was held responsible?