This thesis presents new results on upper limits for the search of Heavy Neutral Leptons (HNL) in the mass range 260-385 MeV decaying to mu-pi pairs using data collected with the MicroBooNE liquid argon time projection chamber operating at Fermilab. The measurement is the first of its kind ever performed in a liquid argon time projection chamber and it uses data collected in 2017 and 2018 with the Fermilab Booster Neutrino Beam, which has an average muon neutrino energy of 800 MeV, using an exposure corresponding to 1.97x10^{20} protons-on-target and a subset of data passing a specific trigger configured to look for possible delayed decays, occurring after the neutrino spill. The results show limits on Majorana Heavy Neutral Leptons in the mass region 260-385 MeV for U_mu^4 in the range [7x10^{-8}, 5x10^{-7}], at the 90% confidence level.