Local officials in Russia say an explosion at an optical plant in the city of Sergiyev Posad, about 70 kilometers outside Moscow, killed one person and injured at least 43 people on August 9.
Five of the injured are in intensive care with serious burns or head injuries, according to the city administration's Telegram account. Some of the 43 people admitted to a regional hospital have shrapnel injuries, it said.
Officials at the city’s central hospital said that a woman had succumbed to wounds sustained in the blast.
Independent Telegram channel Baza shared images of a tall cloud of smoke and identified the site as the Zagorsk Optical and Mechanical Plant, which produces night-vision and other optical devices for the military.
Russian officials later confirmed the location.
Russian news agency TASS has quoted the emergency services as rejecting assertions on social media that the cause of the blast was a drone attack.
That speculation has been fueled by months of remote attacks in Russia that Moscow blames on Kyiv along with Russian reports it "thwarted" two fresh drone attacks near Moscow overnight.
TASS said the blast happened where pyrotechnics were being kept and destroyed a 1,600-square-meter warehouse.
An unexplained fire damaged the same Zagorsk plant in June 2022.
Russian officials have on multiple occasions dismissed credible reports of attacks damaging military or other targets inside Russia since President Vladimir Putin ordered a full-scale invasion of Ukraine in February 2022.
They have described such attacks as a "red line" in the ongoing war, which marks a dramatic escalation of a conflict that began with a covert invasion of Crimea in 2014.
Ukrainian officials have generally avoided publicly confirming any role in suspected drone or other operations that strike across the countries' internationally recognized border.