The primary school in North Belfast at which a gun was discovered will remain closed today.
It was found yesterday afternoon on the grounds of Holy Cross Boys Primary School in the Ardoyne area.
Detectives believe it was to be used by dissident republicans to try and kill police and that it could fire a high calibre round.
District Commander with the PSNI Jonathan Roberts says they have to make sure there is no risk to children in the area.
"At this stage we are not aware of there being anything else but we want to make absolutely sure that there is nothing connected to the gun, any ammunition or any other kind of weapon that may be in the area," said Mr Roberts.
The PSNI say the weapon was found in an area where the youngest children who attend the school would play during the day.
It was discovered by the principal of Holy Cross Boys Primary School while he was helping the caretaker clear an issue with the sewerage system.
School principal Kevin McArevey described the moment he found it as “a scary moment”.
“We had to lift a manhole cover to get the rods down and to my surprise, there was a plastic package just sitting in the sewers,” he told the BBC.
“It was a scary moment for both of us.”
Children and staff were evacuated from the premises as police moved in to secure the area.
They are appealing for anyone with information or who may have noticed anything suspicious in the area recently to contact them.
“I would like to say that this is an act of utmost recklessness and a highly stupid act, the people that have done this do not represent any kind of cause, they do not represent any section of the community," Mr Roberts said yesterday.
“What they have done is endanger the lives of children."