A mistake by an electrical engineer caused Tuesday's massive blackouts across wide swaths of the state, Florida Power & Light announced Friday afternoon.

In an attempt to diagnose a damaged switch at a substation in Miami-Dade County, the engineer disabled two protective devices designed to prevent cascading outages, a violation of the utility's procedures. The engineer, a longtime employee whose name was not disclosed, has been suspended with pay.

The company and the Florida Reliability Coordinating Council, an industry group that enforces federal law, will continue their investigations, which may take months. The Federal Energy Regulatory Commission may also launch an inquiry into what was one of the nation's largest blackouts since an Ohio power line outage in 2003 killed power to millions throughout the northeast.

"Without authorization, the engineer disabled two levels of relay protection," said FPL president Armando Olivera, as he gave details of the company's preliminary findings. "This was done contrary to FPL's standard procedures and established practices."

FPL already has made some interim changes, including providing more oversight of operations and providing "refresher" training to some employees. He declined to detail other changes.

The trouble Tuesday started with a faulty voltage control switch. The engineer inspected the device and, for unknown reasons, removed two protective "relays" that act as circuit breakers. While diagnosing the switch, a short circuit at 1:08 p.m. EST caused the Flagami substation to shut down. The dip in voltage then triggered shutdowns at 38 substations, 26 transmission lines, and generators near Fort Lauderdale.

Olivera clarified that the incident that was reported as a fire produced smoke and sounded like an explosion, but there were no flames.

The engineer "didn't realize the extent and magnitude of the problem," Olivera said. And even if the employee had known, there was nothing he could have done at that point because the shutdowns were automated by computers, he said.

 The blackouts affected 584,000, or 13 percent, of FPL's customers and as many as 3 million people, Olivera said. About 90 percent of FPL customers had their lights back on within two hours.

Olivera said it doesn't appear there were any problems with FPL's infrastructure and maintenance procedures.

But federal regulators and energy experts say the finding of human error doesn't mean FPL is off the hook.

The utility's "frank disclosure helps speed the investigation of this serious incident," FERC Chairman Joseph T. Kelliher said in a statement Friday. "We continue to investigate the blackout to develop a full factual record on the cause or causes of the outage."

Based on FPL's statement, Rutgers University professor Frank Felder said it appears the engineer removed the relay when he knew he shouldn't.

"Overriding relay protection is a serious issue," Felder, director of the university's Center for Energy, Economic & Environmental Policy, wrote in an e-mail. "How was that person able to do so without anyone else knowing about it and preventing it?"

Turkey Point remains closed as FPL conducts mandatory inspections before restarting it and as FPL employees use the opportunity to do maintenance on the plant, said FPL nuclear chief Art Stall.