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The recent arrests of an al Qaeda cell led by Najibullah Zazi, the attempted bombing of the Northwest Airlines jet en route from Amsterdam to Detroit, and the Fort Hood, Texas, shooting rampage, with which Army Maj. Nidal Malik Hasan is charged, all suggest al Qaeda has come close to pulling off mayhem inside the United States.

Adm. Blair’s message was sobering: “Counterterrorism efforts against al Qaeda have put the organization in one of its most difficult positions since the early days of Operation Enduring Freedom [in Afghanistan] in late 2001,” he said. “However, while these efforts have slowed the pace of anti-U.S. planning and hindered progress on new external operations, they have not been sufficient to stop them.”

The testimony specifically warned that al Qaeda is capable of another attack on the United States, marking a change from the 2009 assessment that emphasized the group’s intentions to attack U.S. soil but said their capabilities to launch an attack on the homeland were limited.

Meanwhile, Metro Transit Police on Tuesday conducted an anti-terrorist exercise at a busy underground Metro station.

Mr. Blair testified that al Qaeda is eyeing targets the group in the past attempted to attack, including commercial jets and financial institutions in New York City, and the Washington Metro system.

Mr. Blair’s testimony also focused on al Qaeda’s continuing efforts to obtain biological, chemical and nuclear weapons, but he said he would discuss details only in a closed session.

In testimony after Mr. Blair’s, Mr. Panetta pointed out that the biggest problem for U.S. spies is tracking the “lone wolf” operative who has no background in terrorism.

Mr. Blair said part of the problem is that al Qaeda has switched tactics from spectacular “multiple-cell-based attacks” to smaller-scale operations that are harder to detect.

“The recent successful and attempted attacks represent an evolving threat in which it is even more difficult to identify and track small numbers of terrorists recently recruited and trained and short-term plots than to find and follow terrorist cells engaged in plots that have been ongoing for years,” Mr. Blair said.

Mr. Blair’s written testimony touched on a wide range of topics — from Latin America to the effects of climate change on U.S. strategic interests in the world.

He began his testimony with a stark warning to Congress about the devastating capability of hackers to attack U.S. computer networks.

“Malicious cyber-activity is occurring on an unprecedented scale with extraordinary sophistication,” he said. The director added that the technology today favors hackers and other criminals and not nations to protect their networks.

On Iran, Mr. Blair said the intelligence community suspects that Iran was preparing the groundwork for building nuclear weapons, but that to date Tehran had made no political decision to build the arms. Despite political turmoil that has brought hundreds of thousands of protesters into the streets since the June 12 presidential elections, Iran’s decision-making process would remain the same, he said.

Overall, Mr. Blair said, he gave he protesters little chance for success. “Strengthened conservative control will limit opportunities for reformers to participate in politics or organize opposition,” he stated. “The regime will work to marginalize opposition elites, disrupt or intimidate efforts to organize dissent, and use force to put down unrest.”

The U.S. intelligence community in the past failed to foresee political events in Iran. For example, a noted CIA assessment of Iran in the fall of 1978 predicted there was no prospect for an Islamic revolution. That prediction proved wrong within five months.