SAN FRANCISCO (CBS SF) — Twitter is warning some users they may have been targets of a cyber-attack from a state-sponsored group.

Last week, notices were sent out to a “small group of accounts,” mostly political activists, security researchers in western countries.

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Users were advised to visit the “Tor Project” or the Electronic Frontier Foundation to find anti-surveillance technology and practices.

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Tor is free software that claims to protect users from surveillance by creating a network pathway of encrypted connections. Sources and destinations are harder to trace.