Human error was behind a massive power outage that left hundreds of thousands of customers in the dark and caused a nuclear power plant to shut down, Florida Power & Light Co. said Friday in releasing preliminary findings on the cause.

About 584,000 customers were affected by the outage Tuesday, which occurred when a field engineer working at a substation in Miami disabled two levels of protection without authorization, the power company said.

"This was done contrary to FPL's standard procedures and established practices," the company said in a written statement. "Standard procedures do not permit the simultaneous removal of both levels of protection."

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The outage disrupted operations at 38 substations, one of which serves three of the generation units at the Turkey Point power plant, causing two nuclear units and a natural gas unit to shut down, but safety was never a concern, Florida Power & Light President Armando Olivera said.

"The safeguards built into our system worked as intended," he said. "Because of this, and the experience, training and practice drills of our work force, we were able to restore power to our customers quickly."

Olivera said while the investigation continues, precautionary measures have been taken "out of an abundance of caution" to prevent a recurrence.

The outages were concentrated in the southeast portion of the state, including Miami, but were also reported in the southwestern and northeastern parts of the state as well as in the Florida Keys. In many areas, the power returned before evening, though some transportation facilities and hospitals had to rely on generator power.

The first of Turkey Point's two nuclear power units started operation in 1972. In March 2006, a tiny hole was found in a coolant pipe at the plant. The FBI determined it was vandalism, not sabotage. An out-of-state contractor hired to do routine maintenance was suspected of drilling the hole, the FBI said at the time. The public's health and safety were not at risk, the Nuclear Regulatory Commission said then.

The Associated Press contributed to this report.