Despite decades of intense efforts , many fundamental aspects of Type Ia supernova ( SNe Ia ) remain elusive . One of the major open questions is whether the mass of the exploding white dwarf ( WD ) is close to the Chandrasekhar limit . Here we report the detection of strong K-shell emission from stable Fe-peak elements in the Suzaku X-ray spectrum of the Type Ia supernova remnant ( SNR ) 3C 397 . The high Ni/Fe and Mn/Fe mass ratios ( 0.11–0.24 and 0.018–0.033 , respectively ) in the hot plasma component that dominates the K-shell emission lines indicate a degree of neutronization in the SN ejecta which can only be achieved by electron captures in the dense cores of exploding WDs with a near-Chandrasekhar mass . This suggests a single-degenerate origin for 3C 397 , since Chandrasekhar mass progenitors are expected naturally if the WD accretes mass slowly from a companion . Together with other results supporting the double-degenerate scenario , our work adds to the mounting evidence that both progenitor channels make a significant contribution to the SN Ia rate in star-forming galaxies .