Looking for this particular reference

I am studying this paper: Gravitational radiation from a particle infalling radially into a Schwarzschild black hole by Davis et al.

It deals with the calculation of the energy spectrum of the gravitational waves emitted by a point particle falling radially into the black hole. It says that two methods were used for the calculation of the energy spectrum: a direct integration of the Regge-Wheeler equation and a method devised by Zerilli based on Green’s function. The authors write that details about the first method will be published elsewhere.

I cannot find this elsewhere: can anyone help me finding this reference? (If it has been published at all)