U.S. officials say military investigators are examining a rocket that landed near a U.S. base in Iraq that may have contained mustard gas.
No one was hurt when the rocket landed a few hundred meters from the Qayara West air base on September 21.
The official said several U.S. soldiers inspected the remnants and an initial field test showed it contained a residue of a mustard agent. A second test, however, was negative.
The shrapnel was sent to a laboratory for more testing.
Islamic State militants in parts of occupied Iraq are known to have tried to manufacture chemical weapons.
Earlier this month, U.S. warplanes bombed buildings near Mosul that were used to produce lethal chemicals, including chlorine and mustard gas.
Based on reporting by AP and Reuters