WASHINGTON -- Lawmakers on Thursday pressed an Ohio power company at the center of the blackout investigation about its failure to warn neighboring utilities that it was having transmission line problems just before an eight-state region went dark.

Power grid operators in Michigan have complained they had no warning that problems were developing in Ohio's power lines on the afternoon of Aug. 14, even though a dozen voltage lines in Ohio had failed or were automatically being shut down as a precaution before the blackout.

"Our system was stable, there was no reason to do anything beyond what was done," FirstEnergy Chairman H. Peter Burg told a congressional hearing. He produced a timeline of transmission activities that he said demonstrated that despite the line outages, there was no significant shift in power flow until about two minutes before the blackout began spreading.

"Our system appeared fairly stable until the very end," said Burg, whose Akron, Ohio-based power company -- the nation's fourth largest investor-owned utility company -- has sought to deflect charges that its lines triggered the blackout.

"We strongly believe that such a widespread loss of power could only result from a combination of events, not from a few isolated events," Burg said. He likened the Midwest grid to a boxer going down. "The last punch was important, but the accumulation of all the previous blows led to his weakened condition," the executive said.

But some lawmakers were becoming increasingly convinced that FirstEnergy's problems played at least a significant role in the country's worst power outage, and they wanted to know why alarms were not sounded when the company's lines began to trip.

"You had wires down, but no warning flags up," Rep. Edward Markey, D-Mass., chided Burg.

"Who did you call?" asked Rep. Bart Stupak, D-Mich. "It seems ... there's no accountability when (the high voltage lines) started going haywire."

The loudest complaint about the lack of communication came from operators of power lines in Michigan whose eastern grid became the first casualty in the blackout.

"There was absolutely no warning," said Joseph Welch, president of the International Transmission Co., an independent operator of Michigan's southeastern power grid. "Had Michigan been warned of the problems, a number of actions which would have forestalled the blackout were available."

Officials of Detroit Edison also have complained that a sudden power swing, caused by demand for power in Ohio, came without warning.

Burg acknowledged that "communications systems need to be upgraded," but he argued that information about the Ohio system's performance was available through automatic data communication to the regional grid manager, which monitors power systems in 20 states, and on a computerized "integrated security network."

"We did not receive any information from anywhere," countered Welch, who shared the witness table with Burg and officials from five other blackout-affected utilities at the House Energy and Commerce Committee hearing.

Later, James Torgerson, president of the Midwest Independent Transmission System Operator, disputed claims that his organization had detailed information available about FirstEnergy's grid problems on the afternoon before the blackout.

"We saw a couple of lines that had tripped," said Torgerson, but did not have any details about voltage fluctuations in the system until just shortly before the full force of the blackout hit.

While Michigan was caught by surprise, FirstEnergy's neighboring utility to the south, American Electric Power, was able to protect its system from a cascading power outage.

Seven of the 11 high voltage lines that failed in Ohio during the hour leading up to the blackout either belonged to AEP solely or were shared by AEP with FirstEnergy, according to a timeline provided the House panel by FirstEnergy.

AEP's chairman, E. Linn Draper, said that when power levels increased to certain levels, "our equipment in northern Ohio operated automatically to isolate the problem ... and avoided cascading outages across the AEP system."

He added, "I don't know why all systems didn't perform in a similar manner."

But Welch, the Michigan transmission system operator, said AEP's decision to isolate itself from northern Ohio -- which it did by closing two power lines at 4:06 p.m., two minutes before the blackout began -- may have added to the problem by creating a power void.

AEP officials rejected the scenario. Michigan could have isolated itself as well, suggested Draper when asked Welch's suggestion after the hearing.