Schools across the country have been evacuated due to an "emergency" after threatening emails were sent out on Tuesday morning (May 8).
Schools are being asked to contact police if they receive the emails, although the NCA stressed that "there is no credible threat" within them.
An NCA spokesman said: “We are aware of a number of threatening emails being sent to schools and are investigating alongside our partners in the police.
“We understand that these types of emails can be distressing and while we take the communications extremely seriously we must stress there is no credible threat.”
He said: "I am writing to advise you that this morning I received a malicious email, as did a number of other schools. We have taken advice from the police and they are not treating this as a real threat to the school.
"You will know that unfortunately there have been many such threats to schools across the country.
"However, the police have advised that we should undertake our normal security and safety procedures which we have completed this morning.
"Please be assured that a through (sic) and systematic check of the school has been made and nothing untoward has been found."
Fairholme Primary School in Feltham has been closed today due to the "emergency", according to one social media user. The school office remains closed at 2.30pm.
West Midlands Police have stated a number of the schools in their region are affected but they have not disclosed exactly where they are or the precise number.
Selly Park Technology College in Birmingham was evacuated as a precaution, deputy head Mr Clinton confirmed on Twitter.
He said: "After evacuating the premises following a threat received this morning to a number of schools, and following advice from the police, we have reopened the school and lessons will continue as normal.”
The Metropolitan Police were asked to comment on reports schools in London have also received threats but referred to the National Crime Agency statement.
A spokesman for Warwickshire Police said: “On Tuesday, May 8, police received a number of reports of malicious hoax communications made to educational establishments and schools in the Warwickshire policing area.
“Investigations are ongoing and at this stage, we do not believe there to be a credible threat.”