The  vigilante group known as  Anonymous has claimed credit for yet another of its weekly cyber  attacks, fueling high level concerns among U.S. national security officials that the so-called “hackivists” are posing an increasing threat to government and private sector websites and computer systems.

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“What we’ve seen from Anonymous is alarming,” Michael Leiter, the former director of the National Counter-Terrorism Center and a member of a US. Government advisory board on cybersecurity, said in an interview. “Their  ability to repeatedly take down government websites and potentially penetrate those websites shows that the group is quite committed — and growing in its ability to perpetrate cyber attacks.”

In keeping with its pledge to launch cyber attacks every Friday, hackers associated with Anonymous today took credit for defacing a website associated with InfraGard, an organization that partners with the FBI to share information about potential cybersecurity threats.

“We broke into their webserver, perused their assorted presentation materials, and finally deleted everything and vandalized their website,” according to a statement posted on the Ohio chapter of InfraGard. The group describes itself as an association of businesses, academic institutions and law enforcement agencies that shares intelligence “to prevent hostile acts against the United States.” The hackers in their statement called InfraGard  a “sinister alliance between law enforcement, corporations, and white hat wannabees.” (InfraGard did not immediately respond to a request for comment Friday.)

The attack is the latest in a series of weekly strikes  that appear to have confounded U.S.  cyber security officials and prompted an ever expanding probe by the FBI. These have included temporarily shutting down the CIA’s public web site, penetrating a transatlantic FBI conference call with Scotland Yard about ongoing  investigations into hackers, taking down  the Federal Trade Commission’s website and penetrating the computers of a Washingto area law firm and dumping thousands of its private emails — including some with members of Congress onto the Internet.

In a statement last week, hackers calling themselves AntiSec — an Anonymous affiliate — vowed to  keep up the strikes. “Yes, each and every Friday, we will be launching attacks … with the specific purpose of wiping as many corrupt corporate and government systems off our Internet,” the group stated.

The Wall Street Journal reported this week that Gen. Keith Alexander, the director of the National Security Agency, recently privately warned White House officials that Anonymous hackers could have the ability to cause a limited power outage within the two years through a cyber attack on the U.S. electrical grid. The warnings were reportedly conveyed in a meeting chaired by John Brennan, President Obama’s chief homeland security advisor. (A White House spokeswoman said in an email  she could not discuss “interagency deliberations” but that the “the Administration has made cybersecurity a top priority and we are working tirelessly to protect ourselves from the threats we face.”)

Anonymous has never proclaimed it has such a goal and other national security experts said there is debate about whether private hackers would be able to successfully penetrate the U.s. electrical grid within the next few years.

But Leiter, who is now an NBC News consultant, said the report reflects a “steady drumbeat” of warnings” from the NSA about cyber intrusions in recent years — both by private hackers and even more sophisticated “state actors.”

“There are some similarities to the pre 9/11 warnings from the intelligence community” about a possible terror attack, Leiter said. “People really worry about two things — the theft of enormous amounts of intellectual  property and the economic damage that it can do to the country.”

Moreover, Leiter said, the publicly reported cyber intrusions by groups like Anonymous are dwarfed by an even greater and more ominous hacking attacks that have been discovered b the intelligence community.

“What we are seeing in the public realm is the tip of the iceberg,” he said.

[msnbc]