Although government and energy-industry officials have continued to state that Thursday's massive power blackout was not an act of terrorism, they are unable to rule out the possibility that a computer hacker plunged 50 million people into darkness, a source told Fox News Monday.

There were also reports that Al Qaeda had claimed responsibility for the outage, although U.S. officials said Monday that those claims should be taken with "a giant grain of salt."

A source in the midst of the investigation into the blackout told Fox News there were signs the power grid was becoming unstable as early as Thursday morning.

The final power outage took place at 4:11 p.m. EDT, cascading across lower Michigan, northern Ohio, southern Ontario, most of New York and New Jersey and parts of New England and Pennsylvania in just 9 seconds.

Several 345-kilowatt transmission lines were dropping out of service, for reasons that still are unclear, in the hours leading up to the blackout. A knowledgeable source told Fox News that turning off such a heavy-duty line would be "like shutting down I-95 on the East Coast."

Anatomy of a Power Failure

Around 2 p.m. EDT, a 680-kilowatt line at a coal plant in Eastlake, Ohio, went out of service.

At 3:06, a 345-kilowatt line at FirstEnergy Corp. (search) in Ohio shut down, but the alarm system didn't notify technicians.

At 3:32, pressure from the first downed power line caused another 345-kilowatt line to apparently fail.

At 3:41, two more lines of the same wattage shut down within five minutes of each other.

At 4:10, two more 345-kilowatt lines in Michigan shut down, knocking a coal-fired plant offline.

And at 4:11, a coal-fired plant and a nuclear plant in Ohio shut down, sparking the lightning-quick process that finally turned out the lights on the rest of the eastern Great Lakes and the New York metropolitan area.

By the time the blackout spread outside Ohio and Michigan, wildly fluctuating voltages were overwhelming systems left and right.

Investigators have been increasingly focusing on FirstEnergy, the nation's fourth-largest investor-owned utility. The Akron-based company has 16 power plants, an annual revenue of more than $12 billion and a service area that stretches from Ohio to New Jersey.

FirstEnergy started noticing unusual voltage fluctuations in the Midwest power grid more than three hours before its transmission lines failed. The company said three of its transmission lines in the Cleveland area and the fourth it co-owns failed an hour before the blackout.

The Midwest Independent System Operator (search is also looking at four lines that went down, while the NERC, the utility oversight group, says three lines failed in northern Ohio.

But FirstEnergy said these failures alone couldn't have triggered such a huge outage.

Evidence of that, FirstEnergy said, is that its customers never lost power during earlier failures. Instead, last week's meltdown was indicative, the company said, of a series of breakdowns in the electric grid nationwide.

In the entire history of FirstEnergy, FirstEnergy spokesman Todd Schneider said there hasn't been an outage of any system due to hackers, and that that the weather in northeastern Ohio did more damage.

When asked if the random outages could have been the work of a computer hacker, a source told Fox News: "I'd be lying to you if I told you it was not possible." He added that a hack attack was something he had "been worried about."

Schneider vowed his company would leave no stone unturned in its investigation. He admitted a computer hack was a possibility, but not the leading theory.

The main question is why multiple safeguarding systems failed in their main task to "isolate the failure to one neighborhood," something the mid-Atlantic and New England grids were successful in doing.

Allen Schreiber, chairman of the Ohio Public Utilities Commission (search), told Fox News that the fluctuations were the result of tree contact with the lines in two of the cases, but he stressed that the trees were at heights that meet utility standards. Plus, tree contact wouldn't cause such a catastrophic meltdown.

"I think that when a power line touches a tree, which it does all the time, those power lines go out … every day," Schreiber said. "There's enough back up in the system, enough redundancy, that it should have made no difference at all."

The North American Electric Reliability Council (search), an industry monitoring group, recently passed mandatory cybersecurity regulations for all utilities, but they had not been implemented.

FBI officials said there was no reason to believe that the blackout was the result of a terrorist act, sabotage, hacker, or anything along those lines. While it was "monitoring the situation," it had not launched an investigation.

Possible Al Qaeda Claim

On Monday, the Arabic-language newspaper Al-Hayat ran a story saying a communiqué attributed to Al Qaeda claimed responsibility for the blackout, saying that "the brigades of Abu Fahes Al Masri" had hit two main power plants supplying the eastern region of the United States and major U.S. and Canadian industrial cities.

The communiqué, reportedly posted on the Web site of the International Islamic Media Center (search) and translated by the Middle East Media Research Institute (search), assured that the operation "was carried out on the orders of Usama bin Laden to hit the pillars of the U.S. economy," as "a realization of bin Laden's promise to offer the Iraqi people a present."

The communiqué also said: "Let the criminal Bush and his gang know that the punishment is the result of the action … the Americans lived a black day they will never forget.

"They lived a day of terror and fear ... a state of chaos and confusion where looting and pillaging rampaged the cities, just like the capital of the caliphate Baghdad, and Afghanistan and Palestine were. Let the American people take a sip from the same glass."

The statement outlined certain conditions Al Qaeda purportedly wanted met, including the release of all detainees held by the U.S. and for non-Muslim powers to leave the "land of the Muslims, including Jerusalem and Kashmir."

It noted that the event shut down nine nuclear reactors in New York, New Jersey, Ohio and Michigan and brought public transportation and banking transactions to a halt.

"We tell the people of Afghanistan and Kashmir that the gift of Sheikh Usama bin Laden is on its way to the White House."

High-ranking U.S. officials basically laughed at the idea Al Qaeda could be behind the blackout.

"I would take that one with a huge grain of salt," one official told Fox News.

The source added that there had been no indication, domestically or internationally, that Al Qaeda or anyone else intentionally triggered the power-grid collapse.

"It's probably a good PR stunt to put the idea out there that [Al Qaeda] could do that," terrorism expert Harvey Kushner told Fox News. "Certainly they're interested in having this type of capability, but they prefer, most certainly, a low-tech type of truck bombs, flying planes into buildings. They don't have this type of sophistication."

"Quite frankly," Kushner added, "I wouldn't lose any sleep from the possibility they could take down power grids and things like that."

Fox News' Bret Baier, Jeff Goldblatt, Ian McCaleb, Brian Wilson, Liza Porteus and Anna Stolley contributed to this report.