Washington ? A failure to contain problems with three transmission lines just south of Cleveland was the likely trigger of the nation’s biggest power blackout, a leading investigator said Saturday.

Experts are working to understand why the local line disruptions, some of which occurred an hour before the blackout reached its peak, were not isolated, allowing a cascade of power system shutdowns stretching from Michigan to New York City and into Canada.

“We are fairly certain at this time that the disturbance started in Ohio,” Michehl Gent, head of the North American Electric Reliability Council, said in a statement. “We are now trying to determine why the situation was not brought under control after three transmission lines went out of service.”

Gent said the transmission system was designed to isolate such problems and suggested that human error might have been involved in not containing the situation.

“The system has been designed and rules have been created to prevent this escalation and cascading. It should have stopped,” Gent said.

Later, in a statement suggesting human failings for the events Thursday, Gent said in the future “system operators … will be extremely vigilant” when transmission problems arise.

Energy Secretary Spencer Abraham, who is co-chairman of a U.S.-Canadian task force that will look into the cause of the blackout, said it was still too early to pinpoint a cause.

“We’re not going to prejudge where the problem is,” Abraham told reporters Saturday in Albany, N.Y. “We’re also not going to prematurely leap to conclusions.”

Abraham said the task force was putting together investigative teams that would include experts from the government’s research laboratories as well as private resources. The group’s charge will be to find out what caused the power grid breakdown and recommend actions to prevent a repeat.

Gent did not identify specifically the three power line failures that have become the focus of the investigation. But other council officials said they were among five reported transmission failures in the Cleveland area.

According to NERC, the first report came in at 3:06 p.m. EDT Thursday and involved a 345-volt line that had gone off line. That was followed by reports on other lines failing at 3:32 p.m., 3:41 p.m., 3:46 p.m. and 4:06 p.m.

Two minutes later, according to the NERC summary, “power swings (were) noted in Canada and the U.S.” and three minutes after that power disruptions hit across eight states.