Rumored chlorine gas attack in Iraq raises questions - was the group's stronghold in Syria responsible for other attacks?

The US is investigating reports that terror group Islamic State (ISIS) used chemical weapons against the Iraqi military, after separate accounts from both an unnamed Iraqi Defense official and hospital workers treating the victims told authorities that chlorine gas had been used.

Eleven Iraqi policemen were rushed to hospital last month complaining of dizziness, vomiting, and shortness of breath, the Washington Post reported Friday - all symptoms of chlorine gas poisoning. Yellow gas was also seen emanating from the site near where the policemen fell ill.

This is not the first time ISIS has been linked with chemical weapons attacks, officials said - two other reports making similar accusations were raised last month - but it is the strongest corroboration of facts surrounding an ISIS gas attack thus far.

In June, ISIS seized a weapons complex thought to have held hundreds of tons of lethal sarin and mustard gasses: the al-Muthanna complex, located 60 miles north of Baghdad, which was a central base of former Iraqi dictator Saddam Hussein's chemical weapons program.

While the US State Department waved off rumors that the plant still held chemical weapons, experts stated to the British Telegraph that the chemical residue left at the site could be weaponized regardless - and that ISIS does, in fact, have chemical weapons experience.

In the meantime, Iraqi Defense officials told the Washington Post that the weaponry for the current attack was seized from Iraqi water plants now in ISIS territory, not from the complex.

But the fear remains that this initial attack may only be the beginning, as ISIS trains for larger and larger attacks. Chlorine gas is easy - and legal - to produce and sell, but violates the Chemical Weapons Convention.

“They use it just to create terror,” an Iraqi official said. “But of course we are very concerned.”

Chlorine gas: a Syrian trend?

Last week, the Organization for the Prohibition of Chemical Weapons (OPCW) confirmed that chlorine gas was used in attacks in northern Syria this year - but it is unknown by whom.

The OPCW, which is the international body seeking to implement global laws banning chemical weapons, said it had found "compelling confirmation" of the use of chlorine, in pure or mixed form.

The attacks took place earlier this year in villages where rebels have been fighting government forces, according to the organization, which has been overseeing efforts to destroy Syria's chemical weapons.

Activists in Syria said several months ago that over 100 people had been killed in a chlorine gas attack in the town of Talmenes in Idlib province, and the United Nations Security Council has called for an investigation into the alleged attack. Since then there were reports of other such attacks.

The US has blamed Damascus for the attacks, even though Syria has emphatically denied that it carried out chlorine gas attacks against civilians.

Yet a close look at the initial reports of chemical weapons use in Syria in 2013 reveal that the UN report regarding the attacks never conclusively stated that it was Syrian government forces - and not rebel groups - who used the gas against civilian targets.

Last December, a veteran journalist accused the US of hiding evidence that rebel groups - including ISIS, which began and blossomed in Syria - had also used chemical weapons in the attacks.

Overall, however, the UN has placed the blame squarely on Syrian President Bashar Assad's shoulders - noting in a special panel in September that Assad's atrocities surpassed ISIS's.