A sample of 229618 narrow emission-line galaxies is used to establish two new unambiguous types of evidence for supermassive black holes at the center of their nuclei: (1) the Seyfert 2 galaxies and LINERs follow the same characteristic power law relating the luminosity of the ionized flux with that of the continuum; (2) both show the highest concentration of mass at their center, independent of the morphology of the galaxy, consistent with higher binding energies. The full width at half maximum is shown to be related with the mass concentration, suggesting that the kinetic energy of the gas in AGNs has a gravitational origin. Within the standard accretion model, transition-type objects, Seyfert 2 galaxies and LINERs represent AGNs forming supermassive black holes on different mass-scales, or they could be related through an evolutionary process, the LINERs representing the end product of this evolution.