Martin Goldberg worked at Thorpe Hall school
for 23 years
Indecent images of pupils were discovered on the computer of an Essex deputy head teacher, who was found dead the day after he spoke to officers.
Martin Goldberg, 46, a maths teacher at Thorpe Hall School in Southend, died two weeks ago.
Police found images of boys undressing in the school's changing room, and at a leisure centre, on his computer.
It has emerged officers were told in November 2013 about him buying videos of naked boys.
He was first visited by police on 9 September 2014, but was not arrested because of "insufficient evidence" at the time, an Essex Police spokesman said.
"It is important that we understand why it took nine months for the force to act on this information," Essex's Police and Crime Commissioner Nick Alston said.
"There may be valid reasons, but we deserve an explanation".
Changing room images
The images, both videos and stills, are believed to have been made from 2000 onwards, and "would appear to be of boys aged from nine to 12," police said.
Detectives have identified four children from the footage and have spoken to their parents.
They said there is no evidence of any other offences involving physical contact with the children.
Police said 75 images, which were classed as "indecent", were from the changing rooms at the independently run, co-educational school.
A further 465 were found to have been taken at the leisure centre's swimming pool and 38 from "two other locations".
Officers said in most of the pictures, the children's faces were not visible, or the quality of the image was so poor that they could not be identified.
Images were made of boys at Thorpe Hall School in Southend
Mr Goldberg, a single man who lived alone, had taught at Thorpe Hall school for 23 years.
In a statement, head teacher Andrew Hampton said governors and staff were "very shocked" by the news, and everyone felt "surprised, angry and betrayed".
"There were no indications whatsoever that Mr Goldberg was leading a double life," he said.
"An incident such as this is very alarming and no-one could have foreseen it."
Mr Hampton said an inspection of the school's safeguarding policies in March this year found no improvements were needed.
"Despite this, we are re-enforcing our safeguarding procedures to ensure our provision goes beyond the highest standards expected of all UK schools and are as watertight as it is possible to have them," he said.
Thorpe Hall's head teacher, Andrew Hampton, said there were no indications of Mr Goldberg's "double life"
Southend Borough councillor Anne Jones, responsible for children and learning, said the local authority had been working closely with police and the school.
"The council's primary focus at this time are the children and families directly involved from Thorpe Hall School," she said.
Mrs Jones also said there were "rigorous" safeguarding arrangements in place at Southend Leisure and Tennis Centre, where some of the images were taken.
"Members of the community are encouraged to access the Essex Police helpline on 0800 056 0944 or contact the council's Children's Services should they have any concerns," she added.