A chemical weapons expert and longtime jihadist who joined ISIS was killed last week in a targeted U.S. strike near Mosul, U.S. Central Command announced Friday, dealing a significant blow to what several intelligence sources described as a dangerous program by ISIS to acquire unconventional and illicit weapons.

But it was unclear exactly what roles Abu Malik, also known as Salih Jasim Muhammed Falah al-Sabawi, was playing within ISIS—liaison to former Saddam regime elements, WMD specialist, or both. Nor was it apparent precisely how far along his—and therefore, ISIS’s—chemical weapons plans were.

“He was gathering a lot of equipment—we’re not really sure for what—before we killed him,” a U.S. intelligence official told The Daily Beast. “But it’s concerning that someone who was fairly seriously high up in the [chemical weapons] infrastructure linked up with [ISIS]. This wasn’t some enlisted guy.”

Al-Sabawi was well-known to both American and international intelligence analysts. Pentagon officials told The Daily Beast that he was a mid-level ISIS fighter who provided a means for the Islamic State to develop chemical weapons. Al-Sabawi had worked at Saddam Hussein’s notorious Muthana chemical weapon production facility before he linked up with al Qaeda in Iraq in 2005, CENTCOM said.

A U.S. intelligence source separately confirmed that al-Sabawi had worked at the Muthanna, one of the most significant sites in Iraq’s chemical weapons program, where sarin was produced. But the “big operation”—the “vast majority of agent” produced there—was mustard gas, the source said.

Several U.S. Intelligence sources referred to the prospect of former Saddam regime chemical specialists joining ISIS as a “nightmare” scenario. Last year, Sunni militants occupied the Muthanna facility, which still contained a stockpile of remnants from Saddam's old chemical warfare programs. The New York Times reported that last summer, the Iraqi government informed the United Nations that some 2,500 corroded chemical rockets were still on the grounds of the Muthanna facility, and that intruders had looted equipment there.

A lingering chemical weapons expert roaming “in a place riven with conflict—that’s a real concern,” one American intelligence official told The Daily Beast, referring to the broader Middle East.

A former U.S. official said ISIS’ ambitions to acquire chemical weapons were more than just notional, and while the group had not fully developed a weapon, that possibility was something U.S. intelligence agencies tracked closely for some time.

The announcement of al-Sabawi’s death was the first time that CENTCOM has named one of the 6,000 Islamic State fighters it has killed since U.S. and coalition air strikes began August 8 and suggested that U.S. officials believed they had fended off ISIS’s best resource to develop chemical weapons.

“His death is expected to temporarily degrade and disrupt the terrorist network and diminish ISIL’s ability to potentially produce and use chemical weapons against innocent people,” CENTCOM said in a press release, using an alternate acronym for the group.

Both Iraq and U.S. officials appear to have been pursuing al-Sabawi for weeks. Earlier this month, Iraqi police rounded up several of his close associates. Al-Sabawi was killed on January 24 in a strike that deliberately targeted him, officials said.

The source noted that al-Sabawi’s service in Al Qaeda in Iraq prior to joining ISIS was “a pretty serious situation. He knew a lot about chemical weapons.”

A defense official explained that while there was no evidence al-Sabawi had acquired chemical weapons, “our fear was that he would develop them.”

“His past training and experience provided the terrorist group with expertise to pursue a chemical weapons capability,” CENTCOM said.

After the invasion, the U.S. and its allies promised tens of millions of dollars to redirect Iraqi WMD specialists into peaceful activities beyond they linked up with insurgent or terror outfits. But for five years, those funds failed to materialize. And “five years is a very long time for WMD experts to wait for a program to assist them,” the Arms Control Association noted. In either case, it wouldn¹t have helped in al-Sabawi’s case. He quickly went underground after U.S. forces rolled into Iraq.