Mr. Castor was the Montgomery County District Attorney in 2005 when Ms. Constand approached authorities to complain that Mr. Cosby had sexually assaulted her at his home outside Philadelphia. Mr. Castor declined to bring charges, saying he found “insufficient credible and admissible evidence” and that both sides could be portrayed in “a less than flattering light.”

Ms. Constand then sued Mr. Cosby and won a civil settlement in 2006.

Ms. Constand’s defamation lawsuit referred to comments that Mr. Castor made later, in 2015, after a number of other women had come forward to accuse Mr. Cosby of drugging and sexual assaulting them. Mr. Castor was asked then about his decision not to prosecute and he offered answers that Ms. Constand said suggested she had been inconsistent in describing the assault and had provided a more detailed account in the Cosby civil suit.

“If the allegations in the civil complaint were contained in that detail in her statement to the police, we might have been able to make a case out of it,” Mr. Castor told The Associated Press. In a tweet, he said the account she had given the police was “much different than she told court in her lawsuit,” according to the filing.

Mr. Castor’s successors as prosecutors for Montgomery County decided later in 2015 to bring charges against Mr. Cosby, in part because of new evidence. Mr. Cosby was convicted last year of aggravated indecent assault and is serving a sentence of three to 10 years in prison.

Mr. Castor had argued that his comments were true and so they could not be a basis for a defamation suit.