Over 1.4 million people are suddenly without power in San Diego and surrounding areas in the Southwestern U.S. Thursday, just hours after Homeland Security warned of a possible terror strike in the days leading up to the tenth anniversary of 9/11.

The outage is affecting San Diego and Orange County, Calif. as well as other areas served by San Diego Gas & Electric (SDG&E), including parts of Arizona and Mexico.

We expect power to be out well into the night and into tomorrow in some areas, a spokesman for SDG&E said in a news conference.

The blackout is believed to have been caused by a severing of the transmitter line between Arizona and California.

Essentially we have two connections from the rest of the world: One of from the north and one is to the east. Both connections are severed, said the SDG&E spokesman. Severe heat could have caused the disruption, and the FBI and SDG&E have said that the outage is not the result of a terrorist attack, NBC reported.

U.S. Homeland Security has advised Americans to be on high alert after intelligence has revealed a specific and credible threat of a terrorist attack to coincide with the tenth anniversary of the Sept. 11, 2001 attacks on the World Trade Center and the Pentagon.

Three people are believed to be planning a car-based attack, and at least one of those people may be an American citizen.

As we know from the intelligence gathered from the [bin Laden] raid, Al Qaeda has shown an interest in important dates and anniversaries, such as 9/11, Department of Homeland Security spokesman Matt Chandler said in a statement. In this instance, it's accurate that there is specific, credible but unconfirmed threat information.

In this instance, it's accurate that there is specific, credible but unconfirmed threat information, the department said. As we always do before important dates like the anniversary of 9/11, we will undoubtedly get more reporting in the coming days. Sometimes this reporting is credible and warrants intense focus, other times it lacks credibility and is highly unlikely to be reflective of real plots underway.

Regardless, we take all threat reporting seriously, and we have taken, and will continue to take all steps necessary to mitigate any threats that arise. We continue to ask the American people to remain vigilant as we head into the weekend.

The San Diego Police Department and the U.S. Customs Border Patrol were affected by the power outage, and both are running on backup power.