A unified approach to regular interiors of black holes with smooth matter distributions in the core region is given. The approach is based on a class of Kerr-Schild metrics representing minimal deformations of the Kerr-Newman solution, and allows us to give a common treatment for (charged and uncharged) rotating and nonrotating black holes. It is shown that the requirement of smoothness of the source constrains the structure of the core region in many respects: in particular, for Schwarzschild holes a de Sitter core can be selected, which is surrounded by a smooth shell giving a leading contribution to the total mass of the source. In the rotating, noncharged case the source has a similar structure, taking the form of a (anisotropic and rotating) de Sitter–like core surrounded by a rotating elliptic shell. The Kerr singular ring is regularized by anisotropic matter rotating in the equatorial plane, so that the negative sheet of the Kerr geometry is absent. In the charged case the sources take the form of “bags,” which can have de Sitter or anti–de Sitter interiors and a smooth domain wall boundary, with a tangential stress providing charge confinement. The Arnowitt-Deser-Misner and Tolman relations are used to calculate the total mass of the sources.

Nonstatic, spherically symmetric clusters of counter-rotating particles, of the type first introduced by Einstein, are analyzed here. The initial data space can be parametrized in terms of three arbitrary functions: namely, initial density, velocity, and angular momentum profiles. The final state of collapse, black hole or naked singularity, turns out to depend on the order of the first nonvanishing derivatives of such functions at the center. The work extends recent results by Harada, Iguchi, and Nakao.