Police were first made aware a deputy head teacher may have indecent images of children nine months before they first interviewed him, according to the Essex Police and Crime Commissioner.
Martin Goldberg, 46, a former deputy head teacher at Thorpe Hill School, was found dead at his home on 10 September, one day after he was questioned but not arrested over allegations he purchased indecent images of children online.
Following his death, Essex Police continued their investigation into Goldberg and found he had more than 500 indecent images of children on his computer and other devices that had been recorded on a camera hidden inside a bag.
Police said the hidden camera was used to film boys 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.
Detectives said there is no evidence he physically touched the children or shared the images with anyone else. Both police and Thorpe Hill School said there is no suggestion Goldberg committed the offences with an accomplice.
Nick Alston, Police and Crime Commissioner for Essex, said officers were first made aware of allegations against Goldberg nine months before they went to his home to question him.
Alston said Essex Police received information about Goldberg from police in Toronto in November 2013 but did not informally question him until September this year.
Alston is calling for an explanation why police did not search his home until after his death despite receiving intelligence to suggest he had indecent images of children.
The case has been referred to the Independent Police Complaints Commission (IPCC). Alston said his initial judgment is that something went "badly wrong" in the original assessment of the case.
He said: "I am greatly concerned to learn that evidence gathered by Essex Police strongly suggests that Martin Goldberg was covertly filming boys at various locations, including a public swimming bath.
"I am a parent and a grandparent, and I share the shock, concern and anger that people in our communities are feeling about Goldberg's behaviour.
He added: "I understand the original information from police in Toronto suggesting that Goldberg had purchased videos of naked boys several years ago was received by Essex Police in late November 2013.
"The Police Online Investigation Team first visited Goldberg on September 9, 2014. 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."
Andrew Hampton, headteacher at Thorpe Hall School, said everyone at the school feels "angry and betrayed" at Goldberg's actions and there was no indication he was leading a double life.
He added: "He was a well-regarded teacher who had an exemplary record during his 23 years at the school.
"An incident such as this is very alarming and no one could have foreseen it. Our aim now is to move forward, with the safeguarding of our pupils and the quality of their education remaining our very highest priority."
Hampton said an inspection of the school's safeguarding policy earlier this year found no improvement were needed.
Essex Police said in total there were 75 indecent images from Thorpe Hall School changing rooms, 465 taken from the swimming pool and 38 from the other two locations found at Goldberg's address.
All the boys appear to be aged between nine and 12 and taken from 2000 onwards. Four of the children have been identified and their parents informed.