The U.S. is continuing to investigate an attack this week in northern Iraq that also has indications of a similar chemical agent being used.
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.
The U.S. is still investigating the amount and sophistication of how the agent was delivered.
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.
Samples are also coming in from an attack earlier this week near the town of Makhmour in northern Iraq, a U.S. official said.
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.
Barzani said the coalition official who originally took samples from his men of the chemical substance they were exposed to has called him back to say the samples have been tested and show mustard, but U.S. officials said the samples from Makhmour are still on the way and nothing has been concluded. So far, only the incident in Hasakah is confirmed to be mustard agent.
U.S. Central Command spokesman Col. Pat Ryder said "we don't know what, if anything, may have been used" in northern Iraq.
Secretary of State John Kerry, however, took a more equivocal view of whether ISIS had used chemical weapons.
"We are not surprised by the possibility -- possibility -- that something has been used by somebody, and we have been investigating it very, very closely," he told CNN on his trip to reopen the U.S. Embassy in Cuba. "As I sit here today, we do not have confirmation, No. 1, that it was mustard and, No. 2, that somebody used it."
A doctor at a hospital in Irbil 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.