Claims that timer power units used in the Omagh bombing and a series of other attacks were made by just one person have been heard at Belfast Crown Court.
Twenty-nine people died in the Omagh bombing
Defence lawyers argue there is no scientific basis for the claim and testimony should not be used as evidence in the Omagh bomb trial.
Sean Hoey, 37, from Jonesborough, denies a total of 58 charges.
A forensic scientist said there were similarities between the bomb timers used in the attacks he is accused of.
Dennis McAuley focused particularly on the poor soldering and wiring of electric circuits inside the timer power units.
However, the defence team insist Mr McAuley is not an expert in this area.
During heated exchanges at Thursday's hearing, defence lawyer Orlando Pownall said his evidence was not scientific and claimed it was "just guesswork".
Mr Justice Weir criticised the forensic scientist when he tried to back up his evidence with details of a small study he had carried out using five colleagues.
The details had not been given to either the prosecution or the defence.
The testimony was given during a hearing outside of the main trial.
On Tuesday, the defence will have the opportunity to apply to have the evidence excluded.
Among the charges Mr Hoey faces is the murder of 29 people in the Omagh bomb attack in August 1998.
The trial continues.