Garda technical experts are attempting to track down the person who sent a bomb threat to more than 150 schools on Wednesday.
A Garda source said the investigation assessed the credibility of the email – which is understood to have been attributed to a foreign organised militant group – and are now attempting to find the source.
It was not believed there was any immediate risk to students.
Searches were carried out by gardaí in various schools across the country, though reported army bomb disposal unit operations were related to other incidents.
Some searches continued by members of staff on Thursday before schools opened, but the vast majority were operating as normal upon the advice of gardaí.
While no bombs were found, a source said that making false bomb threats remained a crime under the Criminal Justice (Terrorist Offences) act of 2005.
The threats came on the same day as widespread industrial action over public sector pay, when the majority of schools across the country were closed and pickets placed outside.
The Garda previously confirmed it was “aware of a communication” that was sent to a number of schools on Wednesday and that it was assessing the “contents and credibility” of the emails.
A Defence Forces spokesperson said on Wednesday that the Army Explosive Ordnance Disposal Team “deployed twice today to locations within Dublin” but that the deployments “were routine and of no significance”.