WASHINGTON (AP) — Preliminary tests show traces of the chemical agent sulfur mustard on mortars that Islamic State militants used to attack Kurdish forces in Iraq, a senior U.S. military officer said Friday.

U.S. Brig. Gen. Kevin Killea, chief of staff for military operations in Iraq and Syria, said the field testing was not conclusive, so final tests are underway to learn the full make-up of chemicals on the fragments.

U.S. officials have been looking into reports that IS used the chemical weapon mustard gas in the Aug. 11 attack in Makhmour. Similar reports have surfaced in recent months, including in connection with an attack by Islamic State militants in Syria.

Killea told Pentagon reporters that Kurdish forces brought the mortar fragments to U.S. forces for testing, so there may be questions about the chain of custody of the evidence.

It is unclear where IS might have obtained chemical weapons.

Diplomatic efforts by the U.S. and Russia following a chemical weapons attack in Syria in 2013 led to the removal or destruction of the Syrian government’s chemical weapons stockpiles. But questions remain about whether some of those chemicals escaped destruction or if militants have been able to obtain them in other ways.

U.S. officials have expressed persistent concerns about the possibility that IS would locate and use chemical weapons in its ongoing campaign to take control of more territory across Iraq and Syria.

U.S. and coalition partners are supporting Iraqi and Kurdish forces on the ground in Iraq, largely through airstrikes and a train and equip program. But there have been discussions among top military and defense leaders about using additional U.S. forces in Iraq, possibly to help direct airstrikes or to embed with Iraqi units in order to better train and advise them.

The use of chemical weapons by IS adds an increased element of risk to troops and could further complicate the debate.