The following piece contains explicit descriptions of sexual abuse allegations that may be disturbing to some readers.

(CNN) As the number of women who accuse a former Columbia University gynecologist of sexual assault keeps growing -- it currently stands at 78 -- new evidence has emerged that indicates university officials were warned about his behavior decades ago.

CNN has obtained a letter from a former patient to Columbia officials that spells out her allegations of sex abuse by Dr. Robert Hadden. The year: 1994.

What's more, the head of the hospital's Department of Obstetrics and Gynecology unit responded with a letter to the patient acknowledging receipt and saying her concerns would be addressed-- but the accuser says the official never got back to her, despite his assurance he would do so.

Hadden continued to practice for another 18 years until he was arrested on sexual assault charges.

"I am 37 years old and have had numerous gynecological exams in the past," says the 1994 letter to Columbia, written by Dian Saderup Monson. "I have never, until being examined by Dr. Hadden, been disturbed by the way in which a breast or pelvic exam was conducted."

At a time when the slew of additional accusers has triggered a new investigation into Hadden by the Manhattan District Attorney's office, Monson's case raises questions about how long the university has known about the allegations against Hadden and what steps it took to investigate him.

Dian Saderup Monson

1994 letter evidence of a cover-up, attorney says

Attorney Anthony DiPietro -- who says he is representing Monson and 77 other accusers and is suing Hadden and Columbia University and its affiliated medical centers -- says the letter is evidence of a coverup.

On Friday, DiPietro filed a class-action lawsuit against the university. The five lead class representatives say they were "sexually exploited, abused, harassed and molested at the hands of serial sexual predator defendant Robert Hadden," according to the filing. Two of the women were minors in high school at the time they were treated by Hadden, the suit says. The suit accuses Hadden of "ogling and groping" the minors' unclothed bodies, while engaging "in idle chatter about wholly medically irrelevant information."

DiPietro and the accusers allege in the lawsuit that Columbia knew about Hadden's behavior and protected him for years.

The university "concealed Robert Hadden's sexual abuse for decades," the class-action suit claims, "and continued to grant Robert Hadden unfettered access to vulnerable, unsuspecting" patients, in part to protect its "status amongst other Ivy League institutions ... and (the university's) own corporate and financial interests."

Columbia University did not answer a list of detailed questions from CNN, including what was done about the letter and whether the university has ever done an internal investigation into Hadden.

Instead, it provided this statement: "We are deeply disturbed by the accounts of Robert Hadden's behavior that are now emerging. At the time of Hadden's 2012 arrest, we did not know about the 1994 letter. Had we been aware of it, we would have shared that information with the District Attorney's office. We are fully cooperating with the new investigation and are committed to following the truth wherever it leads."

In 2012, a patient went to the police after an office visit with Hadden, accusing him of licking her vagina during an exam. The arrest was voided and Hadden returned to work and continued to see patients for about another month until he was removed from his post.

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The institution has denied in court filings the civil suit's allegations that the university did nothing to stop the "serial sexual abuse" on "countless occasions."

Hadden's behavior eventually did attract the university's attention -- after his 2012 arrest.

In 2014, Hadden was arrested again, this time following an investigation by the Manhattan DA. The OB-GYN was indicted on nine counts involving six patients who said he abused them. He pleaded guilty to two counts -- criminal sexual act in the third degree and forcible touching -- in a 2016 plea deal that stripped Hadden of his medical license but spared him any prison time.

Hadden's civil attorney did not return calls, but in the civil filings, Hadden, now 61, has denied everything except for the two specific charges to which he pleaded guilty. CNN reached out to Hadden's criminal attorney last week, but she responded saying she no longer represents him.

Hadden, who lives in New Jersey, has not responded to numerous attempts seeking comment.

The doctor's conduct came under renewed scrutiny in January, when Evelyn Yang -- wife of former Democratic presidential candidate and CNN contributor Andrew Yang -- shared her allegations of abuse against Hadden in a CNN interview.

Yang says she was assaulted by the doctor after he returned to work following the 2012 complaint but before being dismissed.

"Can you imagine the audacity of a man who ... continues to do this after being arrested?" Yang said in the interview. "It's like he knew that he wouldn't face any repercussions, that he was protected."

Since Yang's interview aired, more than 40 former patients have come forward to say that they, too, were assaulted by Hadden, DiPietro said.

Accuser was promised immediate 'follow up,' but says she never heard back

Responding to the new allegations, the Manhattan DA opened a new investigation into the former doctor last week.

Monson says Yang's interview brought back the memory of the 1993 appointment. She was also hit with a realization: Columbia University was warned about the gynecologist decades ago.

Monson went looking for the correspondence. A copy of her letter turned up in an old hard drive. She found the response from a Columbia official in a box in her basement.

Her letter, dated May 30, 1994, was addressed to Dr. Harold Fox, the acting chair of the Department of Obstetrics and Gynecology at Columbia-Presbyterian Medical Center (now called NewYork-Presbyterian Hospital/Columbia University Medical Center).

"You may be wondering why it has taken me seven months to write this letter," she wrote. "The chief reason is that I have been too overloaded with the difficulties of pregnancy. ... Secondarily, this isn't a letter I've looked forward to writing. It is not a pleasant prospect to describe on paper an incident that left me, ultimately, feeling violated."

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Monson, who has a PhD in English and American literature, says she also sent a copy of the letter to the medical center's risk management office, hoping to create a record. Her letter to Fox included the following notation at the bottom: "cc: Risk Management Columbia-Presbyterian Medical Center."

Two weeks later, in a letter dated June 14, 1994, Fox thanked Monson for "taking the time to recount the concerns you have regarding the care provided by Dr. Robert Hadden," and promising to "immediately follow up" and to "have a discussion with Dr. Hadden."

"I trust that your pregnancy outcome was excellent," he added.

Fox wrote that he would reach out again in two weeks, but Monson, now 63, says she never heard back from him, the Office of Risk Management or anyone else at Columbia.

She remembers thinking: "Okay, they're not going to take me very seriously. They're not going to go fire this guy."

In an interview with CNN, Monson said that at the time of her appointment with Hadden, she was pregnant with her second child and looking for a new doctor. Her first pregnancy had a number of complications requiring multiple office visits, and she wanted to find a doctor closer to her home in Manhattan. She says a friend who worked at Columbia's renowned hospital system recommended Hadden.

Monson said that what started as a routine first visit with friendly conversation led to a physical examination like no other and ended in an assault.

Monson says that unlike breast cancer screenings she had in the past, Hadden conducted two prolonged and painful exams.

"In past breast exams my physicians have always started at the outside of the breast and, in a circular fashion, palpated inward to the nipple," her 1994 letter to Columbia-Presbyterian Medical Center states. "Dr. Hadden just did a lot of feeling around. Most surprising, he concluded the exam of each breast by pulling very hard on the nipple."

While she was on the exam table for a pelvic exam, Hadden "pressed my knees outward and down so that my genitals were as 'wide open' as possible," the letter says.

"At some point he started masturbating me," Monson told CNN. Monson said when she gasped, Hadden said, "just lubricating the outside" in a manner that made it seem standard. He "began running two fingers up and down my inner labia," she said.

"I've thought about it many, many times since then," she said.

The pap smear she experienced, she wrote in the letter, "took longer and was more uncomfortable than any pap smear I've ever had."

All the while, she wrote, the nurse in the room seemed to be looking away.

"During the breast exam, she had her back half turned to us and, while she did assist during the pap smear (handing Dr. Hadden various swabs, etc.), during the rest of the exam she once again turned away and essentially faced the counter as though she had something engrossing to do. But she had nothing else to do; the countertop was empty, and she was simply standing idly."

It was 'beyond belief,' Monson says

After leaving his office, Monson said she was in shock.

"I couldn't conceive that this person would be molesting me," she told CNN, adding that it was "beyond belief."

Later that night, when she was home, she was finally able to process what happened to her.

"I was sitting on the couch and I was thinking about the exam and it's just like the light bulb went off in my head," she said. "I just suddenly knew it... and I just started sobbing and sobbing."

That night, she told her husband what happened.

Monson decided that she needed to document what happened to her. In a three-page, single-spaced letter, she described what happened during her visit.

She wrote that the appointment left her feeling that "Dr. Hadden's conduct was improper, indeed, grossly so."

"I have tried to imagine any of my past or current physicians giving me the exam he gave me," her letter says, "and I simply cannot."

Columbia officials would not confirm when the university became aware of abuse allegations against Hadden, or answer CNN's questions about whether university officials conducted any investigations prior to the 2012 complaint.

At the time of the 2016 plea deal, Hadden was accused of sexually abusing 19 women. That number has since more than quadrupled.

Fox, through his attorney, declined comment.

"The matter you want to question Dr. Fox about is matter under investigation and about which he is potentially a witness," attorney Susan Necheles said in an email to CNN. "It would be inappropriate for him to speak with the press at this time and he will not be speaking with you."

Monson said she has no idea if anyone at Columbia University ever talked to Hadden after her warning. But she had assumed that the institution would do something with it.

"I thought, this is Columbia -- this is a top-tier university," she said. "I thought, thank heavens this is at Columbia because they will keep this letter. They will put it in a file -- whatever file they keep on their doctors -- it'll be there in a prominent way. It will be marked and surely in the next few years he'll get either verbal or written complaints and they will do something. There'll be an accumulation of some kind of evidence. They'll do something. And I, I just, I felt confident that would happen."