Islamic State propagandists, ever eager to publicize their apocalyptic ideology, have had little to say about the terrorist group’s use of chemical weapons in Iraq and Syria. And they’ve said absolutely nothing about the recent disappearance of radioactive materials in Iraq — materials that could be used in a “dirty bomb.”

Gone missing are 10 grams of Iridium-192, a “highly dangerous” radioactive isotope used to test oil and gas pipelines. The material was stolen last November from a storage facility in the city of al-Zubair. Classified as a Category 2 source of radioactivity, it is considered highly dangerous. Iraqi authorities fear it may have fallen into the hands of ISIS.

Ten grams of radioactive material is not a lot. But it could be added to what ISIS already has on hand. Australian intelligence reported that ISIS already had enough radioactive materials to arm a dirty bomb last June.

The U.S. State Department says it has no evidence that any terrorist group has the missing Iridium-192. But it is also possible that it was stolen by a criminal organization that could sell it to terrorists.

The New York Times this month reported that Belgian police had disrupted a dirty-bomb plot to obtain radioactive materials from one of Belgium’s four nuclear sites. The alleged plot was devised by the same ISIS-linked network behind last November’s Paris bombings.

The good news is that, while detonating a dirty bomb could have a huge psychological effect, its lethality would likely be limited. Far more concerning is ISIS’ use of chemical weapons in Iraq and Syria.

CIA Director John Brennan revealed those attacks earlier this month on CBS’ “60 Minutes.” He also noted that ISIS now has the capability to make small quantities of chlorine and mustard gas.

ISIS’s quest for chemical weapons goes way back. Abu Musab Zarqawi, founder of its forerunner, al-Qaeda in Iraq, experimented with chemical weapons at his training camp in Afghanistan even before the 9/11 attacks. He also was linked to a foiled 2004 plot in Jordan to kill thousands of people with poison gas.

Al-Qaeda, too, has long sought nuclear, chemical, and biological weapons. The group conducted poison gas experiments on dogs in Afghanistan. Osama Bin Laden tried to buy uranium for a nuclear device as early as 1993, and an ambitious al-Qaeda anthrax program was discovered in December 2001.

In a December 1998 interview with Time Magazine, Bin Laden declared that acquiring weapons of mass destruction was a “religious duty.” Today, ISIS is following a similar path.

In November, U.S. and Iraqi intelligence officials warned that ISIS had established a program dedicated to developing chemical weapons with the help of scientists from Iraq, Syria and elsewhere.

These efforts may have gotten a boost in June 2014, when ISIS seized the Al Muthanna State Establishment, the former center of Saddam Hussein’s chemical warfare program. Although the chemical munitions stored there were reportedly decaying and unfit for military use, ISIS may have found a way to transport and use them.

U.S. troops discovered more than 4,500 chemical munitions in Iraq after invading in 2003, although all of them appeared to have been manufactured before the 1991 Gulf War.

It will take relentless work to keep weapons of mass destruction out of the hands of terrorists. The U.S. and its allies must: crack down on the smuggling of weapons and dual-use technologies; conduct sting operations to take buyers and sellers out of circulation, and help vulnerable governments — particularly those in the former Soviet bloc and Pakistan — to bolster security around their nuclear, chemical, and biological facilities.

But the best defense would be to eradicate ISIS and other terrorist organizations as soon as possible, or at least to eliminate sanctuaries where they can develop increasingly dangerous forms of these horrific weapons.

 - James Phillips is the Heritage Foundation’s senior research fellow for Middle Eastern affairs.

 - This piece originally appeared in Boston Herald.