Washington ? As Spanish investigators on Tuesday continued to hunt down leads in the deadly train bombings in Madrid, U.S. officials worried that the attacks marked a shift in terrorism tactics that could prove more difficult to defend against at home.

Numerous small bombs hidden in backpacks — the Madrid bombings involved 10 — are extremely difficult to protect against. Just ask the Israelis, who have seen such smaller-scale bombings become commonplace.

“It’s disturbing,” said Richard Clarke, a former top White House counterterrorism adviser for the Clinton and current Bush administrations. “You can keep a truck bomb out of Grand Central Station. You can’t keep backpacks out.”

If the bombings in Spain are connected to al-Qaida, it’s notable that there were no suicide attackers, a hallmark of Osama bin Laden’s terror network. U.S. Department of Homeland Security officials have warned repeatedly of al-Qaida’s continued interest in hijacking airplanes, prompting officials in the United States and around the globe to shore up airline security. The shift to trains would show that that the terror group had set its sights more broadly and could attack where countries were vulnerable.

“Al-Qaida’s modus operandi is to be flexible, nimble and able to constantly adapt,” said Bruce Hoffman, a terrorism expert for the RAND Corp., a policy think tank.

Al-Qaida started using car and truck bombs and then launched an attack by small boat on the USS Cole in Yemen before moving on to airliners, Hoffman said.

And it has changed how it operates. Knowing that they are often eavesdropped on, al-Qaida operatives have taken to sending important messages by courier and have found ways to move money without leaving a trail for investigators, experts say.

The FBI has long been worried that a suicide bomber could strike on U.S. soil. It’s believed that one reason al-Qaida hasn’t used such a tactic is that its members favor more spectacular attacks with a higher body count.

But Hoffman said al-Qaida and groups affiliated with it, such as Jemaah Islamiyah in Indonesia, seemed to have a new willingness to pursue smaller attacks without a lot of planning. Tracking those kinds of decentralized attacks is more difficult.

“The simplicity of these operations is a guarantee of their ubiquity,” Hoffman said.

In Spain, evidence has been steadily mounting that Islamic extremists were behind the train bombings last week that left 201 commuters dead. Less clear is al-Qaida’s involvement, but there are several signs of at least a loose affiliation.