Footage and images of children undressing in school changing rooms and at a leisure centre were discovered on a computer belonging to a deputy headteacher who was found dead two weeks ago.
Martin Goldberg, of Thorpe Hall School, in Southend, was visited by officers on 9 September amid allegations he had bought material online which may have included images of naked teenage boys.
He was not arrested due to a lack of evidence, an Essex Police spokesman said, adding the following day officers were alerted over concerns for his welfare and he was found dead.
Deputy headteacher Martin Goldberg was found dead on 10 September
An investigation has been launched into why it took police nine months to act over the allegations against Goldberg, of Shoeburyness.
The 46-year-old’s death is not being treated as suspicious.
It is now thought that he had taken images of children at the school and other locations using a hidden camera.
These pictures are believed to date back to 2000 and feature boys aged between nine and 12.
Nick Alston, the county's police and crime commissioner, said early indications were that the case may have highlighted failings.
He added that the original investigation had been launched after police in Toronto, Canada, contacted Essex Police in November 2013 with information suggesting that Goldberg had bought videos of naked boys several years ago.
As a result of this delay, the case has been referred to the Independent Police Complaints Commission (IPCC).
Mr Alston said: "It is important that we understand why it took nine months for the force to act on this information. There may be valid reasons but we deserve an explanation.
"Essex Police has referred aspects of this case to the IPCC, and I expect the independent watchdog to investigate thoroughly and rigorously.
"It would be wrong of me to pre-empt the findings of that investigation, but my initial judgment is that it is likely that something has gone badly wrong here, including possibly in the original assessment and review by CEOP.
"I have also asked Essex Police urgently to review any information or intelligence they possess, such as the material from police in Toronto, about potential paedophile activity in our county and the processes for risk assessment reports of such activity.
"It is essential that our communities have confidence that Essex Police is doing everything in its power to investigate intelligence or information about paedophiles in a rapid and thorough fashion."
Andrew Hampton, headteacher at Thorpe Hall School, said governors and staff felt "shocked, angry and betrayed".
He added: "He was a well-regarded teacher who had an exemplary record during his 23 years at the school and there were no indications whatsoever that Mr Goldberg was leading a double life.
"Our immediate focus is on supporting our pupils, parents and staff at this difficult time. An incident such as this is very alarming and no one could have foreseen it."
He said the school's safeguarding procedures had recently been judged to be "fully compliant" but would be reinforced to go "beyond the highest standards expected in the UK".
A spokesman for Essex Police said: "Detectives have now recovered images that appear to have come from a camera that was hidden in a bag.
"This has been used to film male children undressing in the school's male changing rooms, in the changing rooms at Southend Leisure and Tennis Centre swimming pool in Southend and two other unidentified locations.
"Essex Police understands that this news may be extremely distressing to both parents and children and has set up a special number for people to call if they have any concerns or believe they have information that would assist the investigation."
In total 75 images in the school's male changing rooms, which would be classified as indecent, have been recovered from Goldberg's computer.
There are 465 such images from the swimming pool and 38 from the other two locations.
Detectives have traced four of the children featured in the pictures but the other victims' faces are not visible or the quality of the image is so poor that the children cannot be identified.
The police spokesman added: "It is important to stress that we have found no evidence of any other offences by Mr Goldberg involving these children.
"At this time there is nothing to suggest that he made inappropriate physical contact with any child.
"The school has co-operated with our investigation and we have also found no evidence to suggest that anyone else was involved in Mr Goldberg's criminal activity.
"There is also no information that he shared any of these images or that anyone else was aware of his offending."
The police helpline number is 0800 056 0944, or +44 207 158 0010 for anyone living outside of the UK.