Katz-Bierenbaum was last seen July 7, 1985. According to her husband, Dr. Robert Bierenbaum, she stormed out of their apartment after an argument. Bierenbaum said he and his wife had quarreled during the previous weekend and had been seeing a marriage counselor. Inconsistencies led police to doubt Bierenbaum's story.

Authorities believe that Robert, who is a licensed pilot, murdered Gail in their Manhattan apartment on July 7, 1985. Investigators proved that Robert took a two-hour flight from New Jersey over the Atlantic Ocean the day Gail vanished. Robert failed to mention this fact to authorities during their initial questioning of him. Investigators believe that Robert dismembered Gail's remains in New York, placing her into a duffel bag and then threw the bag into the ocean.

Robert denied the allegations, but was convicted of Gail's murder in October 2000 and sentenced to life in prison in New York.

At a parole hearing in December of 2020, Robert confessed to her murder, claiming that he strangled her, threw her out of an airplane over the ocean. It was the first time he had admitted to the crime. Prosecutors in the 2000 murder trial had presented the theory that he killed his wife in a rage and disposed of her body using his piloting skills, which he confirmed during the hearing, acknowledging he killed her because he was immature and didn't know how to deal with his anger.