The results of that test are not conclusive at this stage, according to Brig. Gen. Kevin Killea, chief of staff for operations for the international coalition against ISIS.
"That is a presumptive field test and it is not conclusive, and what those results tell us is merely the presence of that chemical. It doesn't tell us anything more than that," he said, adding that it would take a couple of week to complete a full test on the fragments to determine what was contained in the mortar rounds and, potentially, its quantity and origin.
Killea did not have any additional information on other reports of chemical agent use in Syria.
CNN reported last Friday that the U.S. was testing samples after Kurdish soldiers arrived at a northern Iraqi hospital suffering from blistered skin and having difficulty breathing.
In another case of apparent chemical weapons use by ISIS, the U.S. government also has test results from an ISIS attack in Hasakah, Syria, from three weeks ago that confirm the terror group used a mustard agent as a weapon, according to several officials.
Officials stress the amount of chemical agent is thought to be small and the concentration low. But it is yet another sign that ISIS is in possession of mustard agent and is using it in battle.
One official said the mustard agent used in Syria is more likely precursor chemicals, rather than a complex munition, a sign this did not come from a cache of Syrian President Bashar al-Assad but was probably mixed by ISIS on its own, using agents or precursor chemicals it obtained.
Precursors are chemicals involved in production stages for toxic chemicals, according to the Organization for the Prohibition of Chemical Weapons.
The officials would not disclose what type of sample was collected.
Typically, forensics could be gathered on the rounds and on the wounds of the victims of the attack. But details of the attack in Syria are scarce, though it is thought to be an attack on Kurdish forces.
In the case of Makhmour, ISIS militants carried out that attack using mortar shells carrying a chemical agent Kurdish fighters had not encountered before, the Peshmerga field commander for Makhmour, Brig. Gen. Sirwan Barzani, told CNN.
A doctor at a hospital in Irbil, Iraq, treating the Peshmerga wounded from the Makhmour incident said there were a total of 15 casualties, some seriously injured, but no fatalities. The doctor did not want to be named discussing a sensitive issue.
U.S. officials are also investigating credible reports of a similar, additional attack that took place in the vicinity of Irbil on Thursday but had few details of what took place.