WASHINGTON -- Wednesday's Oklahoma City car-bombing reflects the widening of a terrorist threat that will test U.S. police and military forces and could challenge traditional American freedoms.
Since Iranian radicals in Tehran took 55 Americans hostage in 1979, America has used law enforcement tactics, diplomacy and, on occasion, bombers and missiles to nab culprits or retaliate against the sponsors of terrorism.
The results have been mixed, but the United States has scored some startling successes, such as the Feb. 7 arrest of Ramzi Ahmed Youssef, alleged mastermind of the 1993 World Trade Center bombing, and the foiling of plots in the Philippines to blow up aircraft and to attack Pope John Paul II.
Law enforcement authorities also boast of preventing 23 acts of terror between 1989 and 1993.
The tools employed in the past may well be inadequate to meet the kind of future terrorism signaled by the poison gas attack in a Japanese subway system.
Some experts predict that new forms of terrorism may make Wednesday's carnage in Oklahoma City pale by comparison.
However, the new methods being considered to combat the modern terrorist peril have alarmed civil libertarians, who fear they may undermine rights guaranteed by the Constitution.
The past decade and a half has seen terrorist groups operating with the support, both material and political, of nations such as Syria, Iran and Libya.
Now, however, the world is seeing groups with only tenuous links to foreign governments or established political organizations.
"These groups operate in a decentralized fashion, unlike many state-sponsored groups, which tend to be more structured," FBI director Louis J. Freeh said in congressional testimony earlier this month.
"The members move between different groups, factions, leaders and objectives based on evolving international or local conditions.
"Americans in any country are vulnerable."
An unreleased Pentagon study, "Terror 2000: The Future Face of Terrorism," warns that within five years, terrorists could launch an attack within the United States with a weapon of mass destruction, using chemical or biological agents, even devices that release deadly radioactivity.
Peter Probst, the 20-year Central Intelligence Agency veteran who co-authored the study, declined to discuss either the study or Pentagon policy.
Giving his personal views, however, he said that politically motivated terrorism is declining and that acts driven by religious extremism or by cults are on the rise.
Political terrorists are restrained by a desire not to offend potential recruits, Mr. Probst said, but the new breed of terrorists don't mind killing or injuring large numbers of people.
And, increasingly, they have access to fearsome devices, like biological agents that could be made in a moderately sophisticated laboratory.
"The marriage of will and means is the great threat of the future," Mr. Probst said.
Mr. Freeh noted in his testimony that ricin, a highly toxic substance that a group called the Patriots Council -- a domestic terrorist group -- had planned to use against law enforcement personnel last year, is synthesized from beans that come from castor-oil plants.
"Sophisticated technology is not required. Neither was it required for the sarin used in Japan, nor the urea nitrate used in the World Trade Center," added Mr. Freeh.
"Many of these deadly components are commercially available."
Over the past decade, the U.S. government has taken a number of steps to strengthen airport security and American facilities overseas, and to better inform the public about terrorist threats.
But James Weidner, a New York lawyer who worked on an anti-terrorist commission set up after the 1988 bombing of Pan Am Flight 103 over Lockerbie, Scotland, said, "Once you start making targets more difficult, they'll go to ones that are easier."
Seeking more powerful law enforcement tools, the Clinton administration and FBI want new laws to restrict nuclear materials and plastic explosives, ban fund raising by terrorist groups and speed deportation of immigrants suspected of terrorism.
And, to prevent future terrorist acts, Americans may have to accept other unpalatable changes.
Law enforcement and intelligence agencies, Mr. Probst said, are "weak in terms of human sources" inside terrorist groups or cults.
The kinds of agents needed "are not going to be Boy Scouts."
"Agents in the inner circle will have blood on their hands," he said.
The CIA has already come under fire for keeping unsavory sources on its payroll.
And the attorney general's guidelines for use of sources in criminal organizations are "very restrictive," Mr. Probst said.
Since the current surge of terrorism began a decade and a half ago, the United States has used military muscle infrequently.
U.S. planes bombed Libya in 1986, in part to retaliate for a terrorist bombing at a Berlin discotheque frequented by American servicemen.
A year earlier, U.S. naval aircraft forced down an Egyptian airliner that was carrying suspects in the Achille Lauro incident.
And President Clinton ordered missile attacks against an Iraqi intelligence headquarters as punishment for an assassination plot against President Bush.
The United States and Britain refrained from using military force in response to the bombing of Flight 103, which killed 270 people, including 243 passengers and 16 crew members, even after investigators identified two Libyan officials as being responsible.
To date, international sanctions have failed to exert enough pressure on Libya to turn over the suspects.
The commission set up after the Lockerbie bombing complained in 1990 that "the United States has too often treated terrorism only as a law-enforcement problem," and it recommended more use of military force.