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US prosecutors are accusing two senior Syrian officials of overseeing a notorious torture center that abused peaceful protesters , including a 26-year-old American woman who was later believed to have been executed.

The indictment was released Monday, two days after a shock rebel offensive overthrew Syrian President Bashar Assad. The US, UN and others accuse him of widespread human rights abuses in a 13-year battle to crush opposition forces seeking his removal from power.

The war, which began as a largely nonviolent popular uprising in 2011, has killed half a million people.

The indictment, filed Nov. 18 in federal court in Chicago, is believed to be the US government's first against what officials say were networks of Assad intelligence services and military branches that detained, tortured and killed thousands of perceived enemies.

It names Jamil Hassan, director of the Syrian air force's intelligence branch, who prosecutors say oversaw a prison and torture center at the Mezzeh air force base in the capital, Damascus, and Abdul Salam Mahmoud, who prosecutors say ran the prison.

Victims included Syrians, Americans and dual citizens, the indictment said. The US-based Syrian Emergency Task Force has long pushed federal prosecutors for action on one case, that of 26-year-old American aid worker Layla Shweikani.

The group presented witnesses who testified of Shweikani's 2016 torture at the prison. Syrian rights groups believe she was later executed at the Saydnaya military prison in the Damascus suburbs.

The whereabouts of the two Syrian officials were not immediately known, and the prospects of bringing them to trial were unclear. Assad's toppling by the rebels over the weekend has scattered his government and left citizens searching prison torture centers around the country for survivors and evidence.