BEIRUT, Lebanon — As the extremists of the Islamic State rampaged across Syria and Iraq, the Twitter user @ShamiWitness was among their most prolific and widely followed English-language supporters. In a near-constant barrage of posts, he cheered the group’s advances, disparaged its enemies and called on Muslims from around the world to heed the call of jihad.

But according to a report broadcast Thursday by Britain’s Channel 4 News, the man behind @ShamiWitness was not an armed fighter, but a cleanshaven Indian executive named Mehdi who lives and works in the city of Bangalore.

The rise and fall of @ShamiWitness, whose Twitter account has since been deleted, illuminates the role of volunteer sympathizers in the global spread of the Islamic State’s message.

The Islamic State, also known as ISIS or ISIL, dedicates significant resources to its social media campaign, regularly releasing photo reports, audio messages and professional-quality videos glorifying the group’s activities. But equally important, analysts say, is the informal network of supporters who may have no official link to the group but magnify its reach.