Most of the suspects arrested in the foiled terrorist plot in London were British citizens — or what some officials refer to as “home-grown” terrorists, according to initial reports on Thursday. The idea that citizens would plot against their own country is a troubling reality, officials say. And while it appears there are significantly more of this home-grown variety in Britain, the United States is not immune.

A home-grown terrorist is generally a citizen or legal resident who can move around the country unrestricted. It is also someone who becomes an extremist from within a country’s borders — such as Timothy McVeigh, the American responsible for the Oklahoma City bombings in 1995.

These people do not generally draw the attention that a foreigner might, says Oliver B. “Buck” Revell, former associate deputy director of the FBI. However, one of the most troubling types of home-grown terrorists is someone who has been to al Qaeda training camps and who organizes cells in the United States, Revell said.

British officials foiled a plot that targeted as many as 10 airplanes flying from London to the United States. Officials say terrorists planned to carry on liquid explosives in plastic bottles.

Department of Homeland Security Secretary Michael Chertoff said the London plot appears to be connected to al Qaeda, but he could not confirm that. Some say the plot has classic al Qaeda hallmarks and is more complex than what home-grown terrorists are capable of.

“Home-grown terrorists don’t have the logistics to assemble operatives,” said Bruce Hoffman, counterterrorism expert with the RAND Corporation.

If there were any concerns of complacency having set in over the past five years, the arrests are a wake-up call that al Qaeda is still on the march, and threats to aviation security are not a threat of the past, Hoffman said.

“The fifth anniversary of the 9/11 attacks takes on an even more urgent and salutary lesson that we can’t afford to be complacent,” Hoffman said.

The United States has raised the threat level to “severe” for all flights from Britain and to “high” for flights to and from all other countries and within the U.S. The Transportation Security Administration has instituted an immediate ban on all carry-on liquids, and some lawmakers are calling for improved explosive screening technology.

But it would not be in the country’s best interest to think terrorists will take the same tactic in the United States as they did elsewhere, said John Rollins, former chief of staff at the Department of Homeland Security’s Information Analysis division.

The home-grown threat is a local and state policing issue, Rollins said. State and local officials will be the ones to notice if anything out of the ordinary is going on in their communities.

Unfortunately, America lacks a culture of information sharing, said former Virginia Gov. James S. Gilmore III who now heads a council on readiness and preparedness. Gilmore said state and local public safety should have more access to national security and national threat information that is often bottlenecked at the federal level.

Eileen Sullivan can be reached at esullivan@cq.com

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