Traces of a component used in Semtex explosives were found in the cells of republican prisoners in Maghaberry Prison, the BBC understands.
The component was detected in swab tests taken during routine searches of two cells at Roe House.
A Prison Service spokesman said no Semtex was found during the operation.
Finlay Spratt, of the Prison officers Association, said it was a worrying development which raised concerns about officer's safety.
"We don't know whether or nor explosives have been brought into the prison, but it is a very worrying development for the staff who already do a very difficult job," he said.
Semtex is manufactured using the chemical compounds PETN and RDX.