Sagittarius (Sgr) is a massive dwarf galaxy in the Milky Way (MW) halo and it has undergone several stripping events. Previous studies were restricted mainly to a few, metal-rich ([Fe/H]~-1) stars that suggested a top-light IMF. Here I present the first high-resolution, very metal-poor stellar sample in the main body of Sgr spanning metallicities from [Fe/H]= -1 to -3. We have derived abundances of 13 elements namely C, Ca, Co, Fe, Sr, Ba, La, Ce, Nd, Eu, Dy, Pb, and Th, where abundances of Sr, Pb, and Th are presented here for the first time. These elements allowed for a determination of stellar ages using nuclear cosmochronology and further provided a challenge to the interpretation based on previous metal-rich studies. The high level of Ca indicates that more massive supernovae (SNe) must have existed and polluted the early ISM of Sgr before it lost its gas. This is in contrast with a top-light IMF with no massive star pollution. Overall, our abundances from Sgr mimic those of the metal-poor halo rather than those representing the metal-rich disc. Our most metal-poor star ([Fe/H]~-3) indicates a pure r-process pollution. Based on star-to-star scatter and abundance patterns, a mixture of low and high mass AGB stars and massive SNe (15-25Mo) are necessary to explain these. Hence, stars stripped from Sgr and similar dwarf galaxies could indeed be building blocks of the MW halo and possibly offer an explanation for metal-poor, s-rich stars in the Galactic halo.