In the past six weeks, Americans have witnessed two jarringly different — but completely accurate — views of al-Qaida’s terrorist network. One image was that of terrorist leaders being hunted down and killed by satellite-guided, pilotless aircraft. The other was of an agile foe slipping past U.S. defenses and increasingly intent on striking inside this country.

New assessments of al-Qaida by the top U.S. counterterrorism experts offer grounds for both optimism and concern a year after President Barack Obama took office. Officials say al-Qaida’s ability to wage mass-casualty terrorism has been undercut by relentless U.S. attacks on the network’s leadership, finances and training camps. But even in its weakened state, the group has shifted tactics to focus on small-scale operations that are far harder to detect and disrupt, analysts say.

The deadly November shooting rampage at Fort Hood, Texas, and the failed Christmas Day attempt to bomb an airliner — both examples of the low-tech approach — have raised the fear level in Washington and across the country. Some terrorism experts say the worst could be still to come as a wounded jihadist movement thrashes about in search of a victory.

“The noose is tightening, and al-Qaida’s leadership is accelerating efforts that were probably in place anyway,” said Andy Johnson, former staff director of the Senate Intelligence Committee and now national security director for the Washington think tank Third Way.

In the past year, Johnson said, the “good guys have been scoring the points,” killing key al-Qaida leaders and disrupting multiple plots. But pressure on al-Qaida in Iraq and Pakistan has forced terrorist operatives to flee to new havens, such as Yemen, and step up the search for weaknesses in Western defenses. While battered, “the enemy is unwavering and determined,” he said.

The U.S. ability to strike al-Qaida’s nerve center was on display recently with news of the apparent death of the leader of the Pakistani Taliban, a close ally to al-Qaida in the lawless frontier along the Pakistan-Afghanistan border. Hakimullah Mehsud, who suffered severe injuries in a missile strike in mid-January, was the second leader of the group to find himself in the path of a CIA Predator aircraft in the past six months. He also was closely linked to the Dec. 30 suicide bombing that killed seven CIA officers and contractors in Afghanistan’s eastern Khost province.

U.S. drones have struck al-Qaida and Taliban targets inside Pakistan 12 times this year, putting the Obama administration on a course to surpass 2009’s record-setting 53 strikes, according to a tally by the Web site Long War Journal.

In testimony before two congressional panels last week, top U.S. intelligence officials said the campaign has shaken al-Qaida’s core leadership, the small band of hardened terrorists led by Osama bin Laden. The attacks, combined with a successful squeeze on al-Qaida’s cash supply, have impeded the group’s ability to launch ambitious, complex terrorist operations on the scale of the Sept. 11, 2001, strikes on the World Trade Center and the Pentagon, the officials said.

But intelligence officials also warned lawmakers of worrisome new evidence of al-Qaida’s ability to adapt. In an annual “threat assessment” to Congress, spy agencies described the emerging threat as more geographically dispersed and also low-tech, favoring lone operatives and conventional explosives.