Spectrophotometry of SN 1996al carried out throughout 15 years is presented .
The early photometry suggests that SN 1996al is a Linear type-II supernova , with an absolute peak of M _ { V } \sim - 18.2 mag .
Early spectra present broad , asymmetric Balmer emissions , with super-imposed narrow lines with P-Cygni profile , and He I features with asymmetric , broad emission components .
The analysis of the line profiles shows that the H and He broad components form in the same region of the ejecta .
By day +142 , the H \alpha profile dramatically changes : the narrow P-Cygni profile disappears , and the H \alpha is fitted by three emission components , that will be detected over the remaining 15 yrs of the SN monitoring campaign .
Instead , the He I emissions become progressively narrower and symmetric .
A sudden increase in flux of all He I lines is observed between 300 and 600 days .
Models show that the supernova luminosity is sustained by the interaction of low mass ( \sim 1.15 M _ { \odot } ) ejecta , expelled in a low kinetic energy ( \sim 1.6 \times 10 ^ { 50 } erg ) explosion , with highly asymmetric circumstellar medium .
The detection of H \alpha emission in pre-explosion archive images suggests that the progenitor was most likely a massive star ( \sim 25 M _ { \odot } ZAMS ) that had lost a large fraction of its hydrogen envelope before explosion , and was hence embedded in a H-rich cocoon .
The low-mass ejecta and modest kinetic energy of the explosion are explained with massive fallback of material into the compact remnant , a 7 - 8 M _ { \odot } black hole .