When Waymo tests its self-driving car prototypes on public roads, teams of workers sit inside control rooms remotely monitoring their every move. Some of these workers—Waymo calls them “guardian angels”—are ready to push buttons that bring vehicles to a halt when safety concerns arise.

For five months in 2018 though, Waymo employed an individual as one of these guardians who was in federal prison five years ago and has a history of mental illness and substance abuse, The Information has learned. Among his past troubles, the worker previously sought to disrupt California’s power grid by shutting down a data center operated by his then-employer,  which managed the state’s electricity market, he admitted in a guilty plea in court. In recent weeks, Waymo, a unit of Google parent company Alphabet, fired the worker, telling some employees he posed a safety risk, a person briefed about it said.

The hiring of the worker, along with previously unreported issues involving Waymo backup drivers and recent interviews with former employees, raise fresh questions about the vetting of safety personnel by autonomous vehicle operators. The reliability of these workers could determine how quickly Waymo can expand its nascent robo-taxi service, which it is currently testing in several suburbs of Phoenix. Interviews with the people who have worked at the company also provide a rare window into the tools “remote assist dispatchers”—the job title of workers such as the recently dismissed contractor—use to try to prevent mishaps involving Waymo vehicles.