Abstract: The L→ll′l′νlνL decays (L=τ,μ; l,l′=μ,e) in the Standard Model (SM) and in the effective field theory (EFT) description of the weak charged current are studied at low energy, both for polarized and unpolarized L, keeping for the first time l,l′ mass dependence. The discrepancy is clarified between two previous SM calculations improving their precision. The recent 35σ anomaly found in τ→eγeντ could be checked using our precise prediction for the τ→eee eντ decays, which shall be measured analysing already existing data from the first generation B-factories. It is shown how measurements of the di-l′ mass distribution (with appropriate cuts) and T-asymmetries are able to reveal the corresponding lepton flavor violating (LFV) processes without neutrinos in the final state.