The watchdog revealed that after the incident, Metropolitan Police placed their three-month-old son in a database with information on children known to police.
The incident has drawn renewed attention to the use of stop and search powers in Britain amid Black Lives Matter over the summer.
Ms Williams, a GB team sprinter, and Portuguese 400m record holder Mr Dos Santos, 25, were dragged out of the car and handcuffed in footage shared widely on social media. No illegal items were found.
Days after the incident, the Metropolitan Police Commissioner said two reviews of the Force Professional Standards Directorate did not reveal any misconduct on the part of his officers.
Dame Cressida denied that the images in the evidence “reveal racism” and said the vehicle was stopped because of the way it was driven, after apologizing to Ms Williams.
On Thursday, the IOPC Fund announced that it was investigating five officers into potential violations of police standards, including the use of force, authority, respect and courtesy.
The watchdog seeks to know “whether Mrs. Williams and Mr. Dos Santos were treated less favorably because of their race”, the accuracy of the accounts provided by the police officers and if the statements made by the Metropolitan Police during the ‘investigation were appropriate.
The IOPC Fund is evaluating “the way in which certain agents initially approached Mr. Dos Santos” and the decision to handcuff him and keep him under control after a search.
Investigators are investigating why Ms Williams was grabbed before police asked her to cooperate with the search, was handcuffed and restrained after the search.
They also seek to know why the vehicle was tracked and stopped, whether the search was “reasonable and objective” and whether the force used was “lawful, necessary, reasonable and proportionate”.
The IOPC Fund said the couple’s baby, who was in the back of the car at the time, had been added to Scotland Yard’s Merlin database for children known to police and wondered if there were “legitimate reasons”.
Regional Director Sal Naseem said: “We have decided that this meets the threshold for a malpractice investigation, the allegations will now be the subject of a full and independent investigation.”
Scotland Yard has confirmed that nothing was found during the search and that no arrests have been made.
He said his Professional Standards Branch had looked at all available footage and found “no indication that the actions of the police would constitute a violation of police professional standards of conduct serious enough to warrant disciplinary proceedings.”
Dame Cressida Dick apologizes to athlete Bianca Williams for her ‘distress’ after stop and search
Metropolitan Police said they referred to the IOPC Fund after Ms Williams complained about their treatment on social media.
A spokesperson said that in an initial response to the IOPC Fund, the force said, “If proven, the breaches would only constitute minor violations of professional behavior standards or performance issues.”
Metropolitan Police said they would cooperate fully with the ongoing investigation, but did not suspend any of the officers involved or assign them to restricted duties.