WASHINGTON — The massive Northeast blackout on Aug. 14 was a "preventable" failure, a U.S.-Canadian government investigation said Wednesday, concluding that a major Ohio utility violated guidelines meant to protect the power grid and didn’t know that its control room computer systems had stopped working two hours before the final outage.

The three-month-long task force investigation, headed by the Department of Energy and its Canadian counterpart, accuses FirstEnergy Corp. of Akron of failing to warn other control centers on the grid until it was too late.

The report also says FirstEnergy’s control room operators were not adequately trained and that the company failed to trim trees low enough to keep its transmission lines from hitting limbs, causing short-circuits that tripped out crucial power lines on its system.

One power line after another went out in a chain reaction, and with each outage the heavy volume of power flowing through Ohio toward Canada and the U.S. East Coast built up on remaining lines until finally the overload became unmanageable.

Shortly after 4:10 p.m., huge power surges swept through Michigan and then into New York, knocking out power from Manhattan to Toronto to Detroit.

Unlike other grid control centers, FirstEnergy’s does not have a computerized map board showing conditions on major lines and at power plants — another weakness in its system, the report said.

Outside analysts studying the blackout have questioned whether FirstEnergy should have seen that its systems were malfunctioning and at least alerted others on the grid.

Charles Jones, FirstEnergy’s senior vice president for delivery, said the company realized its computers weren’t working as intended after about an hour.

"But we had no knowledge of the extent of it," Jones said. The company sent operators to power substations to read meters on flows and report the information by telephone, but by then the situation was deteriorating too quickly.