Police have launched an investigation after letters containing threats were posted to three schools.

On Tuesday April 16, malicious letters were received by Watford Grammar School for Girls, Bromet Primary School, and Ascot Road Community Free School.

Hertfordshire Constabulary says it is increasing its presence around the three schools to reassurance communities and the schools have activated their safeguarding procedures.

But police say there is no evidence to suggest threats contained within the letter are legitimate.

Details of what was written in the letter have not been revealed.

Parents and guardians at the schools have all been made aware of the situation, say police, and the force says it is taking the incidents “extremely seriously”.

Police have made one arrest in connection with the investigation.

Detective Inspector Jason Finnegan, from the Watford Local Crime Unit, said: “In the past week we have followed several lines of enquiry, and so far, there is no evidence to suggest that the threats contained within the letters are legitimate.

“We are continuing our work to determine the source of these letters in order to deal with the offender(s) in a robust manner.

“Students, parents and the local community will see increased police presence around the three schools in question as a reassurance measure. We understand that some people may find this unsettling however if you have any concerns then please do not hesitate to approach an officer.”

Police are appealing for anyone with any information to come forward.

Anyone with information should contact Detective Inspector Jason Finnegan in the Watford Local Crime Unit by calling 01923 472961 or emailing jason.finnegan@herts.pnn.police.uk or you can contact Crimestoppers anonymously on 0800555111.

• A 38-year-old man from St Albans has been arrested in connection with the investigation and released under investigation while enquiries continue.