WASHINGTON — Late this spring, Iraqi forces fighting the Islamic State in Mosul discovered three unfired rocket-propelled grenades with an unusual feature — a heavy liquid sloshing inside their warheads. Tests later found that the warheads contained a crude blister agent resembling sulfur mustard, a banned chemical weapon intended to burn a victim’s skin and respiratory tract.

The improvised chemical rockets were the latest in a procession of weapons developed by the Islamic State during a jihadist arms-manufacturing spree without recent analogue.

Irregular fighting forces, with limited access to global arms markets, routinely manufacture their own weapons. But the Islamic State took the practice to new levels, with outputs “unlike anything we’ve ever seen” from a nonstate force, said Solomon H. Black, a State Department official who tracks and analyzes weapons.

Humanitarian de-miners, former military explosive ordnance disposal technicians and arms analysts working in areas captured from the Islamic State provided The New York Times with dozens of reports and scores of photographs and drawings detailing weapons that the militant organization has developed since 2014, when it established a self-declared caliphate in Syria and Iraq.

