Opportunities were lost to break up a child abuse ring when information about two girls being exploited in Huddersfield was not followed up, according to a report.
A review into the cases of more than 20 girls in Huddersfield said that nothing relating to child sexual exploitation (CSE) was flagged up in the majority of cases more than a decade ago.
The review, published on Monday, said this may have been because social work professionals were not looking for CSE at that time.
Social work academic Mark Peel looked at files relating to 22 girls involved in the prosecution of 20 men who were given lengthy jail sentences last year.
Dr Peel concluded that in the cases of Girl 4 and Girl 8, Kirklees Children's Services officers "knew at the time that these young women most likely to have been engaged in inappropriate, exploitative and illegal sexual activity to the extent that they had sufficient evidence to conclude these vulnerable young women were at risk of 'serious harm'".
He said: "In both instances however it would appear that, other than recording this information, no subsequent preventative safeguarding action was taken, and that thus an opportunity to break the CSE ring operating in Kirklees, and protect these girls directly and others more generally, was lost."
Dr Peel said 15 of the 22 teenagers were known to social services but, with the exception of Girl 4 and Girl 8, their files did not contain information "to come to the reasonable conclusion that the young women were at risk of sexual exploitation".
He added: "In retrospect it is tempting to ask why didn't they spot CSE when it was so obvious?
"The simple fact here is that nobody spotted it because we simply weren't looking."
He said: "The case files do not corroborate 'stories' carried in the press that 'everyone knew' that CSE was widespread and established.
"Indeed, I have not found a single instance across all the case files reviewed of agencies voicing and sharing any such concern."
But he said one reference in the files of Girl 4 said: "She is being exploited into prostitution, she hangs around with a number of men who take her money. She is a very promiscuous girl."
Dr Peel said: "As we know that Girl 4 was one of the 22 girls featuring in the CSE court case, it is possible that if remedial action had been taken in 2007 to safeguard her that links to others suffering similar exploitation might have been established, and the operation of the CSE ring discovered."
Twenty men jailed last year embarked on a "campaign of rape and other sexual abuse" against vulnerable teenage girls in Huddersfield, a judge said.
Ringleader Amere Singh Dhaliwal, 35, was jailed for life and told he must serve a minimum of 18 years by a judge who said: "Your treatment of these girls was inhuman."
Dr Peel's review was published on Monday alongside another commissioned by KirkleesSafeguarding Children Board which focused on current policies, procedures and working practices.
Mel Meggs, Kirklees Council director of children's services, said: "Dr Peel has been clear that the vast majority of cases were handled in line with the policy and practice of the time.
"However, the studies of Girl 4 and Girl 8 show that, historically, professionals did not always spot the signs of exploitation and did not always respond appropriately to concerns. We are truly sorry that these two girls were not protected in the right way.
"We are far from unique in experiencing CSE in our district, but we continue to leave no stone unturned."
Jacqui Gedman, Kirklees Council chief executive, said: "We absolutely agree with Dr Peel that a small number of the cases could and should have been handled differently at the time, and on behalf of the council I want to apologise to the girls that we let down.
"This is a common theme in reviews of historic cases around the country and we must all ensure that we learn from the past.
"We now have a much greater understanding of the risks and issues involved in CSE and we can be confident that the progress of recent years would lead to very different actions today."