By Sam Frizell
March 2, 2015

Twitter is investigating threats made against its employees by people claiming ties to the Islamic State of Iraq and Greater Syria (ISIS).

A message appeared online Sunday calling on ISIS supporters to kill Twitter employees, apparently in response to the company’s efforts to block ISIS-related accounts.

“You started this failed war,” reads one post in Arabic. “We told you from the beginning it’s not your war, but you didn’t get it and kept closing our accounts on Twitter, but we will soon come back.”

One message singled out Twitter co-founder Jack Dorsey in particular, showing an image of crosshairs overlaid on Dorsey’s face. Dorsey is now CEO of mobile payments company Square.

“Our security team is investigating the veracity of these threats with relevant law enforcement officials,” said Twitter spokesman Jim Prosser.

ISIS has often used Twitter and other social media to broadcast its message, publish video of violent acts and recruit new followers. The group has shown a penchant for “gaming” Twitter by using automated accounts to make its online supporter base seem larger than it likely actually is.

Meanwhile, Twitter regularly deletes posts and suspends accounts showing executions or violent actions. The company’s terms of service ban posting “direct, specific threats of violence against others.”

Contact us at editors@time.com.

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