What justification does the Ahmadiyya movement have that Prophet Muhammad wasn't the last prophet?

I was wondering how it is acceptable for the Ahmadiyya movement with not considering Prophet Muhammad the last prophet when that seems to be so central to Islam? I thought the belief that Prophet Muhammad was the last prophet was one of the five pillars? I am looking for scripture references from the Ahmadiyya movement that helps me understand how they justify it. Did the question never come up? I am not looking for answers that indicate why the Ahmadiyya movement is blasphemous, incorrect, or non-Islamic.