The New York Post reported the prosecution rested its case against Marshall, 86, Tuesday after presenting 72 witnesses during 17 weeks of testimony.
Among those who testified in the Manhattan Supreme Court trial were healthcare staff and providers who cared for Astor before her 2007 death at the age of 105.
Marshall is facing conspiracy and grand larceny for allegedly strong-arming his ailing mother into signing over more than $60 million in bequests before she died. His lawyer, Francis Morrissey, 66, also is on trial.
A handwriting expert testified for the defense Tuesday that Brooke Astor, who had Alzheimer's disease, "very probably" signed an update to her will that prosecutors contend was forged, the New York Daily News reported.
"I believe this is an authentic signature," Alan Robillard, a former FBI documents examiner, told the jury.
The trial, which started in April, is expected to take a two-week break next week and not reach final arguments until sometime in September.