It's not as if more evidence is needed. But last month's killing in El Salvador of 13 people, including four United States marines and two American businessmen, shows again the limits of retaliation against terrorism.
Despite an intensive US-backed investigation into the murders, sources inside and outside the government say the chances of finding and apprehending the terrorists remain small.
The four unarmed marines and nine other civilians were killed when a rebel unit, part of the leftist Farabundo Mart'i National Liberation Front (FMLN), opened fire on a caf'e in the fashionable Zona Rosa district in San Salvador. Fifteen others were wounded in the shooting.
The rebel attack was the worst in the capital in four years and the latest episode in a new wave of urban terrorism directed at the government of Salvadorean President Jos'e Napole'on Duarte.
US officials say they would like to use the Salvador killings to demonstrate the ability of the US, working with a friendly government, to bring terrorists to justice.
Since jurisdiction lies with El Salvador, the San Salvadorean government will lead the investigation. Accordingly, the government has mounted an unprecedented nationwide search for the terrorists, featuring posters with artists renderings of the suspects and periodic radio broadcasts asking for information from the public.
But the US is playing a major supporting role in the investigation.
Immediately following the incident, President Reagan called for the accelerated delivery of $128 million in US military aid appropriated this year. He also ordered an executive-branch review of possible additional aid, including police cars, radios, and other equipment needed to strengthen the law enforcement capabilities of the Salvadorean government.
The President has also directed various US law enforcement agencies to give ``whatever assistance is necessary'' to apprehend the gunmen. State Department sources say such help is likely to come in the form of technical support.
Leading the investigation is a unique Special Investigations Unit, trained under the aegis of the State Department and authorized under a special congressional exemption to a law that generally prohibits direct US aid to foreign police units.
``It's a kind of Salvadorean FBI,'' says a State Department source. Initially set up to deal with El Salvador's right-wing death squads, the new unit, which was trained last year at a FBI facility in Puerto Rico, is now focusing its efforts on leftist terrorism.
US sources also confirm that a US-trained urban antiterrorist unit, attached to the Salvadorean chief of staff, is taking part in the investigation.
Despite the unprecedented levels of US assistance, few here hold out much hope that the investigation will be successful.
``There are limits to what can be done,'' says a State Department official. ``We know who did it. We even know where they're located. But finding the specific individuals is another matter.''
``The incident has been properly described by the administration as a barbaric act,'' says former US Ambassador to El Salvador Robert White. ``But the idea that somehow you can go out and arrest revolutionaries and get evidence against them seems unrealistic.''
A Salvadorean Embassy spokesman here describes the investigation as ``very high priority, involving the highest levels of the government of El Salvador, including President Duarte.''
But the emphasis placed on the investigation has been criticized in some political quarters of El Salvador as an effort on the part of the Duarte government to play to an American government.
Critics in El Salvador complain there have been more than 50,000 casualties in the civil war, many at the hands of right-wing death squads and the Salvadorean Army. But the only crimes that get this kind of high-level attention, they say, are the ones involving Americans.
``This kind of thing goes on here every day. But, suddenly, four Americans are killed and we discover there's a war going on,'' one Salvadorean says.
In a strong statement delivered at a recent Sunday homily, Salvadorean Archbishop Arturo Rivera y Damas condemned the attack. But he added, ``all human life has value, not only the citizens of one specific nation.''