Abu Malik was killed in an airstrike Saturday near Mosul, the U.S. Central Command said in a statement. The statement did not give the circumstances of Malik’s death, whether he was the target of an airstrike or explain why it took nearly a week to announce that he’d been killed, Washington Times reports.

Malik’s death was the first time Central Command has reported the killing of a specific individual in more than 900 airstrikes on ISIS targets since August.

The statement described Malik, who was also known as Salih Jasim Muhammed Falah al Sabawi, as a chemical weapons engineer who’d worked at Saddam’s Muthana chemical weapons production facility in Anbar province, the country’s premier chemical and biological research facility until it was largely dismantled after the U.S.-led Persian Gulf War of 1991.

The ISIS captured the facility in July, but U.S. officials insisted then that whatever chemical stores remained there had been rendered useless by U.N. inspectors years ago.

Still, the prospect that the ISIS might be able to develop a chemical weapons capability has worried officials.

The statement said that Malik had joined al Qaida in Iraq, the ISIS’s predecessor organization, in 2005 – another reminder of the close links between the ISIS and al Qaida in Iraq, which was the target of the so-called “surge” of U.S. troops in 2007 that U.S. officials had credited with pacifying the country.

It was unclear how critical a figure Malik was in the ISIS’s organization, however. The statement said his “past training and experience” provided “expertise to pursue a chemical weapons capability” and that his death would “temporarily degrade and disrupt the terrorist network.”