Many commentators at first suspected the bombing was the work of Middle Eastern fundamentalists. But it eventually came to light that it was the work of homegrown terrorists.
The chief perpetrator McVeigh, who had fought in the first Gulf War, said later that one of his primary motives was the storming of the Branch Davidian compound at Waco on 19 April 1993 -- exactly two years before the Oklahoma bombing.
Nowadays, the United States’ militant militia movement -- which tends to be anti-central government, anticommunist, and racist -- has been overshadowed by the threat of Al-Qaeda.
In 2002, U.S. federal agents arrested Jose Padilla, a U.S. citizen, at an airport in the central city of Chicago. The Justice Department says he is an Al-Qaeda operative and was planning to find a good site to explode a so-called dirty bomb in the United States. Officially declared an enemy combatant, Padilla has had only limited access to his lawyer and is being held indefinitely.
A few months later, federal agents in Texas arrested William Krar, who was found to have a bomb like the one used in Oklahoma City, as well as a half-million rounds of ammunition. Krar is now serving an 11-year prison term.
Padilla has no record of militant activity and had no weapon when he was arrested. Krar was known as a right-wing zealot and was heavily armed.
One analyst says that even though their numbers have been dwindling, Americans should be just as concerned about domestic terror groups as they are about Al-Qaeda.
The disparity in their treatment indicates a double standard, according to Daniel Levitas, the author of "The Terrorist Next Door," which studies indigenous American terrorist movements. He told RFE/RL that he attributes this double standard simply to the government saving face.
"I think it's embarrassing to the United States to present frightening evidence that there are people in this country who are just as fanatical and murderous as Islamic terrorists halfway around the world," he said. "Because in the view of some in the Justice Department, [domestic terrorism] somehow undermines the moral authority of the United States to condemn Al-Qaeda."
As a result, Levitas said, the Justice Department made little of Krar's arrest, but took pains to publicize Padilla's. In fact, he notes that then Attorney General John Ashcroft interrupted a trip to Russia to announce the Padilla arrest.
Levitas said that even though their numbers have been dwindling, Americans should be just as concerned about domestic terror groups as they are about Al-Qaeda.
Before the Oklahoma City bombing, Levitas said, there were as many as 20,000 active members of hundreds of militias in the United States. Since that attack, and those of 11 September 2001, their numbers have dropped dramatically to perhaps a few thousand.
Levitas said that many members quit the militia movement either because they were not committed to the level of violence demanded by the groups, or because of the crackdowns on terror movements that followed the two attacks. Yet with fewer members, the U.S. militia movement is now even more dangerous. "Those people who stayed in the militia movement are far more radical than those who were in the movement before," Levitas said.
Levitas added that those still in the movement don't need organizations to support them or large paramilitary units to strike. He points to the few men needed to mount powerful attacks in Oklahoma City, or later in Washington and New York.
That argument doesn't convince James Starrs, a longtime law-enforcement specialist who is now a professor of law and forensic sciences at George Washington University in Washington. He told RFE/RL that the militias’ declining membership is accompanied by crippling internal discord.
"The reports that we have lately received indicate that there's a lot of fractioning of the homegrown terrorists and that there is a great disparity among their allegiances. I think, if anything, the homegrown terrorists are diminishing in number, diminishing in capacity, and diminishing in [their] ability to carry forward acts of terrorism," Starrs said.
Starrs said that however the American public may view the militant militia movement, what is important is how law enforcement treats them. He concedes that they couldn't stop the Oklahoma City attack, but he said the Krar case shows they aren't ignoring the problem.
"We've had people like that in our society and managed to cope with them for all these many years. I don't think we need any further special precautions than we already have," Starrs said.
But Starrs was quick to add that this does not mean he approves the way the government is handling Padilla. He said he opposes any infringements on any prisoner, whether an American or a foreign citizen. Indefinite confinement without trial is "anathema to the American way."