NEW YORK (WABC) -- A former Columbia University gynecologist was indicted Wednesday by a federal grand jury for allegedly sexually abusing patients.
Robert Hadden reached a plea deal in 2016 with the Manhattan District Attorney's Office that allowed him to avoid prison time after he was accused of sexually abusing 19 patients.
A judge granted him the lowest sex offender status as part of the deal, in which he surrendered his medical license.
The indictment charges him with abusing six victims, including a minor, but says between 1993 and 2012 Hadden abused "dozens" of his patients, including "multiple minors." None of the victims was identified by name.
"Hadden did so through a process that entailed developing a relationship with his victims and causing them to trust him, before engaging in a course of increasingly abusive conduct, which Hadden attempted to mask under the guise of legitimate medical care," the indictment said.
Federal prosecutors outlined a series of breast exams and pelvic exams that were "excessively long and sexualized" after ensuring no one else was present in the exam room.
According to the indictment, Hadden frequently targeted women who were young and unlikely to have much, if any experience with another OB/GYN.
"For many victims, Hadden was their first gynecologist, and for others Hadden was their doctor during their first pregnancy," the indictment said. "In doing so, Hadden intentionally targeted victims who would not know what to expect during their exams."
The Manhattan District Attorney's Office reopened the case in February and issued the following statement after the federal indictment was announced.
"Our office provided substantial assistance leading to today's indictment, and our continuing investigation - which examines potential failures by Dr. Hadden's employer and hospital to disclose additional incidents of abuse to our office and to regulators when required - is intensely active and ongoing."
Related: Manhattan district attorney reopens sex abuse case against gynecologist
At least 78 patients now claim they were sexually assaulted by Hadden, including the wife of former presidential candidate Andrew Yang.
The allegations renewed calls from elected officials and women's rights groups that DA Cy Vance resign over his handling of the case.
Vance has not yet announced if he will seek another term.
Evelyn Yang told CNN earlier this year that Hadden assaulted her in 2012, including when she was seven months pregnant, and said Hadden's punishment amounted to a "slap on the wrist."
"What happened to me should have never happened," she said.
The sexual abuse allegations against Hadden date until as early as 1993. A lawsuit brought by more than two dozen of accusers says he groped and penetrated patients during vaginal examinations and "mole checks" that served "no medical purpose."
Hadden also made sexually inappropriate remarks and surreptitiously perform oral sex on patients, the lawsuit says, "to satisfy his own prurient and deviant sexual desires."
Related: Senate passes legislation aimed at protecting Olympic athletes from assault
Marissa Hoechstetter, another Hadden accuser, has said Vance's office misled her about the statute of limitations in Hadden's case and was already negotiating the plea deal when she was still talking to prosecutors about testifying at a potential trial.
Hoechstetter said she initially felt relief sitting in court watching Hadden plead guilty, but realized after reading the agreement that prosecutors agreed not to charge him with any other crimes they were aware of - including the ones she said he committed against her.
"I just realized how badly I have been treated and taken advantage of," Hoechstetter said.
(The Associated Press contributed to this report)
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NYC gynecologist charged with abusing patients, including Andrew Yang's wife