Five teenagers were arrested in London on terrorism charges on Monday following a series of raids, police said.
“The arrests relate to plans to travel to join a proscribed organisation,” the note added.
A spokesman for the police would not confirm where the teenagers were planning on travelling to or which banned group they had intended to join.
The operation followed searches of four homes last month and a further raid in south London on Monday.
In 2015 three teenage girls from east London went missing and were believed to have run off to join the Islamic State group in Syria.