Hackers aiming to call attention to the dangers of mass surveillance said they were able to peer into hospitals, schools, factories, jails and corporate offices after they broke into the systems of a security-camera startup.

That California startup, Verkada, said Wednesday it is investigating the scope of the breach, first reported by Bloomberg, and has notified law enforcement and its customers.

Swiss hacker Tillie Kottmann, a member of the group that calls itself APT-69420 Arson Cats, described it in an online chat with The Associated Press as a small collective of "primarily queer hackers,...