In the summer of 2014, as his followers were ravaging the cities of northern Iraq, Islamic State leader Abu Bakr al-Baghdadi convened a secret meeting with a weapons expert whose unusual skills the terrorist chief was anxious to acquire.

His guest was a small man, barely above 5 feet tall, and he had only recently been freed from a years-long stint in U.S. and Iraqi prisons. But before that, Salih al-Sabawi had been an Iraqi official of some renown: a Russian-trained engineer who had once helped President Saddam Hussein build his extensive arsenal of chemical weapons.

Baghdadi had summoned Sabawi, 52, to offer him a job. If supplied with the right equipment and resources, could he produce the same weapons for the Islamic State? Sabawi’s reply, according to a later intelligence report about the meeting, was yes. He could do that and more.

Thus began what U.S. and Iraqi Kurdish officials describe as a crash effort aimed at building the biggest arsenal of chemical and, potentially, biological weapons ever assembled by a terrorist group. Within six months, under Sabawi’s direction, the Islamic State would manufacture mustard gas, a chemical weapon from the World War I era, as well as bombs and rockets filled with chlorine.

But Sabawi’s ambitions, and by extension Baghdadi’s, were much broader, according to newly disclosed details on the Islamic State weapons program. Iraqi Kurdish intelligence reports, seen by The Washington Post, shed new light on the role played by Sabawi, a mysterious figure known within the terrorist group as Abu Malik, and the ambitious plan by Islamic State leaders to develop and use weapons of mass destruction in Iraq and abroad.

New insights also are emerging from a U.N. investigation that is combing through millions of pages of Islamic State records as it seeks evidence of the group’s war crimes. In addition, several current and former U.S. officials in interviews with The Post spoke for the first time in detail about an urgently planned military operation, conducted in 2015 by U.S. Special Operations forces with assistance from Kurdish Peshmerga operatives, to kill Sabawi and crush the weapons program before it reached maturity.

U.S. officials learned through electronic surveillance in 2014 that Sabawi was working to produce powerful new weapons using highly lethal botulinum toxin and ricin, while also pursuing plans to make weaponized anthrax. Botulinum toxin, a neurotoxin derived from same bacteria that causes botulism, was explored as potential weapon by both the United States and the Soviet Union during the Cold War. Ricin, a toxin extracted from castor beans, was weaponized by the Soviets and used in political assassinations.

Sabawi’s intention, current and former U.S. officials said, was to create a large stockpile consisting of multiple types of chemical and biological agents to be used in military campaigns as well as in terrorist attacks against the major cities of Europe.

“They were specifically looking at Western Europe,” a senior U.S. official knowledgeable about Islamic State operations said. Like several other U.S. and Iraqi officials, he spoke on the condition of anonymity to discuss details obtained from sensitive surveillance operations. “We know they were also interested in U.S. military bases, on the continent, or really anywhere,” the official said. “They were ultimately going to go with the easiest target.”

That the Islamic State had manufactured small quantities of chemical weapons has been previously reported. The terrorist group used chlorine and mustard gas against Kurdish and Iraqi forces nearly two dozen times, from early 2015 until the liberation of the Iraqi city of Mosul two years later.

Other terrorist groups, including al-Qaeda, have explored the feasibility of making chemical and biological weapons. But by recruiting Sabawi, the Islamic State had acquired the services of a rare expert with years of practical experience in making chemical weapons in industrial-sized quantities.

The U.S. government’s only public reference to Sabawi came in a brief 2015 Pentagon statement announcing the recent death of a “chemical weapons engineer” named Abu Malik in an airstrike. Few knew at the time about the extent of Sabawi’s experience or his vision for providing Islamic State leaders with frightening weapons to augment the group’s terror campaign in Europe.

“If Abu Malik had survived, his experience working for Saddam’s program would have made the threat of the Islamic State’s chemical weapons much higher,” said Gregory Koblentz, an expert on chemical and biological weapons and director of the Biodefense Graduate Program at the Schar School of Policy and Government at George Mason University. “It is pretty horrifying to think of what could have happened if the Islamic State had used a chemical weapon, instead of guns and bombs, to conduct one of their attacks in a major European city.”