From the day in March that prosecutors announced charges against 50 people in a sweeping college admissions fraud investigation, they have held out a tantalizing mystery: an unnamed family that they said had paid the college consultant at the center of the scheme $6.5 million — far more than any of the parents named in the case — to get their child into college.

The student is Yusi Zhao, who was admitted to Stanford in 2017, according to a person with direct knowledge of the investigation. Neither she nor her parents, who live in Beijing, have been charged, and it is unclear whether they are currently being investigated. Stanford rescinded Ms. Zhao’s admission in April, and she is no longer a student there.

The person with knowledge of the inquiry said that Ms. Zhao’s family was introduced to the college consultant, William Singer, by a financial adviser at Morgan Stanley based in Pasadena, named Michael Wu. A spokeswoman for Morgan Stanley said that Mr. Wu had been terminated for not cooperating with an internal investigation into the matter and that the firm was cooperating with the authorities. Mr. Wu did not respond to a phone call.

At a court hearing in March, the lead prosecutor in the admissions case, Eric S. Rosen, said that Mr. Singer had tried to get Ms. Zhao — whom Mr. Rosen did not identify by name — recruited to the Stanford sailing team and created a false profile of her supposed sailing achievements.