WASHINGTON — (CNN) The American-led military coalition against ISIS said Tuesday that a final laboratory test of the weapon ISIS fired at a US-Iraqi base in northern Iraq last week did not contain a mustard agent as earlier believed.

Initial field tests of what has been described as either a rocket or artillery shell did indeed test positive for the chemical weapon and US personnel were required to undergo decontamination.

Last week, the Chairman of the Joint Chiefs of Staff, Gen. Joseph Dunford, told the Senate Armed Services Committee that “we assess it to be a sulfur mustard blister agent” when asked about the attack.

Military officials had told CNN that a second test of the weapon had produced negative results while a third lab test was inconclusive.

The most recent test announced by Dorrian represents the final test of the weapon.

A US defense official told CNN that ISIS has carried out dozens of chemical weapons attacks against civilians and US allies in northern Iraq and Syria, and that the US military considers itself to be operating in a chemical environment.

The official added that while the latest test of a chemical weapon proved negative, he said he expected such attacks to happen in the future.

Dunford said the military has targeted over 30 ISIS installations involved in the production of chemical weapons.

US Navy Capt. Jeff Davis, a Pentagon spokesman, told reporters Monday that the US has issued 50,000 gas masks to the Iraqi forces in order to counter ISIS chemical weapons attacks.