OKLAHOMA CITY - There is a reflecting pool now where the horrifying shell of the bombed federal building once stood, and a museum displays photographs of the 168 people killed that April day 10 years ago, along with small reminders of the lives they lived - a young woman's red lipstick, a doctor's stethoscope, a baby's pacifier.
But the remembrances of what happened at the Alfred P. Murrah building on April 19, 1995, which until Sept. 11, 2001, stood as the worst act of terrorism on American soil, do not only look back. In offices adjacent to the memorial museum, a small group of researchers work on how to prevent, and better respond to, the next attack.
Their work has urgency. Independent groups that monitor extremist activity inside the United States say that while the country has focused since 2001 on the threat from foreign terrorists, domestic operatives like Oklahoma City bomber Timothy McVeigh have not gone away and, in some ways, are more dangerous than ever.
For officials at the National Memorial Institute for the Prevention of Terrorism, it is the sad fact inherent in work intended to honor the victims of the Oklahoma City bombing. What happened here, they know, could happen again.
"It will happen," said Ken Thompson, the institute's external affairs director. Thompson's mother was killed in the blast at the Murrah building, and he now works in an office overlooking the memorial pool and the rows of sculptural chairs representing each victim.
"I don't know that anyone can expect that the federal government can stop 100 percent of the attacks," he said. "We know it will happen again, and the most important thing we can do is be better prepared to respond and bring the people to justice than we have in the past."
The institute's research offers a reminder of Oklahoma City's unique lesson about the danger of the homegrown terrorist. Its extensive "Terrorism Knowledge Base" shows that, of 25 terrorist incidents recorded in the United States from 2003 through January, all but three were thought to have been carried out by militant domestic ecological groups, such as the Earth Liberation Front.
The remaining incidents involved the mailing of ricin-laced letters to government offices in Washington in late 2003 and early 2004. Those cases are unsolved, but they also are suspected to be the work of a domestic criminal who complained in the letters about new Department of Transportation trucking rules and used the signature "Fallen Angel."
More broadly, the Southern Poverty Law Center counted 751 active hate groups operating inside the United States in 2003, the most recent year for which figures are available. The FBI last year identified ecological extremism as the top domestic terrorist threat, responsible for more than 1,100 criminal acts and $110 million in damage since the mid-1970s. Mark Pitcavage, director of fact finding for the Anti-Defamation League, said that in the decade since the Oklahoma City bombing, 15 law enforcement officers have been killed by anti-government extremists or white supremacists.
"There's a lot of activity, and some of it's quite serious," Pitcavage said in a recent interview. "My one concern is that people not go to sleep on this issue. We may have other threats from international extremism. But that doesn't mean domestic terrorism is no longer a problem."
Pitcavage and others who track domestic terrorism say the threat has changed over the past 10 years. The anti-government militia movement, which drew the greatest scrutiny after Oklahoma City, shrank after its association with the ugly carnage of that attack. Dire warnings from militia leaders that Jan. 1, 2000, would be marked by catastrophic computer failures hurt the movement as well when the predictions proved untrue.
Three of the country's most prominent white supremacist groups - the National Alliance, Aryan Nations and the Creativity Movement - also have suffered setbacks because of the death or incarceration of key leaders. But observers say that the loose-knit followers who remain could pose a greater threat, one compounded by the widespread use of the Internet, which extremist groups use heavily for recruitment, fund raising and mobilization and which was not a factor a decade ago.
Daniel Levitas, the Atlanta-based author of The Terrorist Next Door: The Militia Movement and the Radical Right, said that while the movement is smaller than it was 10 years ago, "the people who remain are much more hardened in their beliefs and in their capacity for violence."
"In the wake of Sept. 11, I think most people have forgotten the fact that there are Americans who are just as fanatical and maniacal as al-Qaida," Levitas said. "People have short memories."
Across the country, cases that have both attracted great attention and gone almost unnoticed demonstrate the continuing threat of U.S. extremism:
When he pleaded guilty last week to a string of bombings that included the deadly attack at Centennial Olympic Park during the 1996 Summer Games in Atlanta and blasts outside abortion clinics and a gay nightclub, Eric Rudolph also issued an unrepentant, 11-page, typewritten statement in which he quoted the Bible extensively and derided the Olympics as a plan to promote "global socialism."
"The purpose of the [Olympic] attack on July 27th  was to confound, anger and embarrass the Washington government in the eyes of the world for its abominable sanctioning of abortion on demand," Rudolph wrote.
As part of a plea deal that allowed Rudolph, 38, to avoid the death penalty, authorities said the former fugitive, who evaded law enforcement for more than five years, revealed the location of more than 250 pounds of dynamite he had buried in western North Carolina, along with a fully constructed bomb twice as powerful as the one he exploded during the Atlanta Olympics.
In a case from Tennessee, the FBI last fall arrested 39-year-old Demetrius "Van" Crocker in a sting operation in which Crocker allegedly attempted to buy sarin nerve gas and a block of C-4 explosive from an undercover agent.
According to an arrest affidavit, Crocker told the agent that he admired Adolf Hitler and thought that building a concentration camp for Jewish insurance executives would be a "desirable endeavor." A government informant also said Crocker had an "absolute hatred" for the U.S. government and wanted to "build a bomb to be detonated at a government building, particularly a courthouse, either federal or state," the affidavit said.
Even Oklahoma City has seen more recent incidents of U.S.-bred terror, federal authorities say. A self-described leader of the Aryan Nations, Sean M. Gillespie, was charged last year with attempting to firebomb a local synagogue.
According to court records, Gillespie previously was arrested for disrupting a Martin Luther King Day rally in Spokane, Wash., and was discharged from U.S. Army basic training at Fort Benning, Ga., after he told a sergeant he wanted out because he "had gained all the knowledge he needed while undergoing basic training."
FBI Deputy Assistant Director for Counterterrorism John E. Lewis said in an interview that counterterrorism remains the bureau's top priority, and it does not differentiate the threats from domestic and international terrorism.
The lessons of the Sept. 11 attacks and the Oklahoma City bombing, Lewis said, "were hard learned, and they have not been forgotten."
Lewis and outside analysts, such as Pitcavage, credit the formation of joint terrorism task forces with detecting a number of domestic extremist plots before they could be executed. In many parts of the country, Pitcavage said, "you can scour the earth for Islamic extremists, and you won't really find much."
But even that heightened effort by law enforcement can be stymied by someone like Mc- Veigh or Rudolph - whom Lewis called "the lone wolf" character - who operates independent of organized groups and generally avoids the public arena before acting.
A retired FBI agent who helped work the bureau's command center at Oklahoma City in the days after the attack said in an interview that threats from such people might not be preventable.
"That's just a vulnerability of a democratic society," said I.C. Smith, who retired as head of the FBI's Arkansas field office in 1998 and wrote the book Inside: A Top G-Man Exposes Spies, Lies and Bureaucratic Bungling Inside the FBI. "Terrorism is cyclical. I think it's inevitable that at some point, domestic terrorism will once again rear its ugly head, and some people might argue that the conditions, I wouldn't say are ripe right now, but certainly are leaning that way."
Mark Potok, director of the Intelligence Project for the Southern Poverty Law Center, said it is "inevitable that we're going to suffer something like Oklahoma City again."
"It's worth remembering that before Oklahoma City, from the right and the left, terrorism was quite targeted. McVeigh was the first one to carry out a mass murder that targeted the general population," Potok said. "That changed the whole climate. And I'm afraid it really changed it permanently. To make a big splash now, you have to kill 169."
Still, in Oklahoma City, at the site that remains the worst incident of domestic terrorism, there is a determination to defy the predictions.
Ken Thompson's mother, Virginia Thompson, worked in a credit union office inside the Murrah federal building. She was 56 when she was killed in the April 1995 blast. Her body, buried under rubble, was not recovered until after the building was imploded.
After working at a credit union himself, then as a manager at a car dealership, Thompson went to work at the terrorism research institute, which was created as the last component of the three-part memorial project to remember the victims of the bombing.
"The reason the entire institute was started is because family members and survivors here in Oklahoma City did not want other communities to feel the same pain that we had," he said recently as sunshine flooded the memorial field. "Sometimes you think that 10 years is such a long time ago, and sometimes it's a blink of the eye."
With 14 staff members and an annual budget from the federal government of about $15 million, the institute has launched a range of projects since its start in 2000. The terrorism database records unclassified information on more than 20,000 incidents around the world since 1968 and can be accessed by the public at the institute's Web site, www. mipt.org.
The institute also has created two restricted-access databases for emergency responders that evaluate equipment and lessons learned from other terrorism incidents. And it has funded a range of research projects, including a study of anthrax vaccinations started before the 2001 attacks and a study of protective clothing for first responders that included researchers from the Johns Hopkins University.
"We were tilling the ground and doing the hard work that needed to be done prior to 9/11, and then 9/11 just accelerated everything," retired Army Gen. Dennis J. Reimer, the institute's director, said. "I think it's probably fair to say that 9/11, because of the magnitude of what happened, probably overshadows what happened here. But I think it's always good to remind people that this was the first attack, and we need to remember what happened here."