Chief of Detectives Dermot Shea says an internal probe of the detective's conduct began a week ago.
A message left on a phone used by the detective in the past wasn't returned.
The dismissed charge concerns an alleged sexual assault of an aspiring actress by Weinstein in 2004.
Details emerged on Thursday after the court unsealed a letter that Manhattan prosecutors wrote to Weinstein's lawyer, Benjamin Brafman, in September.
The revelation of numerous allegations about Weinstein, beyond the three women involved in the criminal case, helped spark the #MeToo movement against sexual harassment and sexual assault.
The witness had given the different account to a NY police officer who had failed to tell prosecutors, the letter added. The D.A.'s office said it "recently learned" the witness" account was shared with police and that police "failed to inform our office of important account prior to the indictment in this case'.
It says the accuser told prosecutors that the inconsistencies may have been caused by a flawed memory.
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An attorney for Weinstein has said that he plans to file motions to dismiss each of the remaining charges.
Brafman said outside court that "this is not about victim shaming".
The defense said it would also subpoena police interview records after claiming an NYPD detective "may have unfairly tainted these proceedings" when he prepared Evans for her testimony.
In a statement, a lawyer for Evans said that she was disappointed by the district attorney's decision to "abandon" her client. "Nor does it reflect on Lucia's consistent allegation that she was sexually assaulted with force by Harvey Weinstein", lawyer Carrie Goldberg said.
"The people can not oppose the defense request for the dismissal of that single count", Illuzzi said. "The evidence shows that the criminal case against him is strong". He has denied all allegations of non-consensual sex.
The ditched sexual assault count stemmed from allegations by Lucia Evans, who told the New Yorker a year ago that Weinstein forced her to perform oral sex in his office in 2004.
Weinstein says he is innocent.
The New York Police Department is also investigating claims that the lead detective on her case had advised Evans to keep this information that could potentially undermine her claim to herself.
He added: "The case against Mr Weinstein is, in my view, not sustainable".
The development was announced in court with Weinstein looking on.
US-based tabloid website TMZ reported last week that a casting director claimed Evans once told her she performed oral sex on Weinstein to gain an acting role.
According to the witness, who was not named in the court filing, Evans had been drinking and "appeared to be upset, embarrassed and shaking" when she told the story.