WEST PALM BEACH, Fla. (AP) Utility company investigators will likely look beyond a substation fire to determine the cause of a recent blackout that left as many as 3 million Floridians without power, energy experts say.

Florida Power & Light officials said Wednesday they are uncertain how long it will take to learn why a small fire at a Miami-area substation wasn't contained and how it triggered an automatic shutdown of two nuclear reactors. They have not yet ruled out human error.

The rolling power outage began at about 1 p.m. Tuesday when a switch short-circuited at the substation, causing a small fire and taking down two power distribution lines between Miami and Daytona Beach.

In response to the disturbances in the power grid, the two nuclear reactors at FPL's Turkey Point facility near Miami automatically shut down, eventually leaving nearly a fifth of Florida's population without power.

Investigators probably will try to determine whether the nuclear reactors shut down prematurely, said Vijay Vittal, an electrical engineering professor at Arizona State University.

"Just the fire and the switch alone were probably not the cause of such a large outage," he said.

Federal guidelines require electrical grids be able to maintain capacity even when one system goes down, such as a substation, a protection needed to prevent cascading blackouts, Vittal said.

But once the reactors shut down and demand exceeds power output, the lights start going out across the state, he said.

The reactors remained off-line Wednesday afternoon. FPL officials would not say when they would be back online, citing company policy. However, the Nuclear Regulatory Commission, which had inspectors at the faulty substation, said the reactors would likely remain down for up to 24 hours.

One lingering question is why the problem wasn't contained at the substation.

George Gross, a professor of electrical and computer engineering at the University of Illinois at Urbana-Champaign, compared it to dominoes.

"As one pushes the other, they all fall in line," said Gross, who participated in a 2002 Energy Department study of ways to improve the delivery of electric power in the nation.

"One of the things to keep in mind is that electricity travels at the speed of light so all it takes is one little problem and it propagates very quickly," he said.

Gross said it was too soon to speculate on what might have caused the problems in Florida.

"We know there was a fire, but a fire by itself should not have provoked this kind of action," Gross said. "I think there are some other factors that we're just not hearing about yet."

Copyright 2008 The Associated Press.