Washington ? On the heels of the second terrorist strike in two weeks on the London mass transit system, terrorism experts had some words of comfort for jittery Americans: It’s less likely to happen here.

“I wouldn’t say it’s inevitable in the United States,” said Juliette Kayyem, co-director of Harvard University’s project on U.S. legal strategy to combat terrorism and an expert on homeland security.

“Part of it are the homegrown elements abroad, with the militancy of the clerics, which are allowed to flourish in ways they’re not allowed to flourish in the United States,” said Kayyem, who has advised the federal government on terrorism and national security.

Jim Carafano, a homeland security analyst and 25-year Army veteran who testifies frequently to Congress on countering terrorism, also sees Americans as safer than Europeans. He said that “terrorists perceive the United States as a harder target, and the terrorists are going after the things that are easier, just to show they can commit terror.”

While they are quick to say that the threat of attacks on trains and buses – like those recently in London – exists here, people with expertise in terrorism and homeland security add that America poses a tougher target because of other factors as well:

Muslim populations in Europe tend to be large and concentrated, helping newcomers or homegrown militants blend in.

Large groups of Muslims in Europe feel disenfranchised and European governments tend to tolerate calls for jihad, which is rarely seen in the United States.

The blow dealt to al-Qaida by the war in Afghanistan and other actions has forced the terror group to rely on locally sponsored attacks by loosely affiliated groups, as opposed to strikes planned by al-Qaida leaders, and such groups are at the very least less visible in the United States.

Over time, terrorism expert Daniel Byman warns: “This sort of mid-level attack is likely, just because it’s easy to do, there’s an open set of targets, it’s very hard to prevent and America remains at the top of the list for terrorists around the world.” Byman works at the Brookings Institution and Georgetown University and has worked with the government on intelligence and terrorism issues.

But what works in America’s favor at present in Byman’s view “is that there doesn’t seem to be a local group to do the attack itself or to provide logistics or support.”