Originally published March 15, 2004, as an op-ed in the Orange County Register.

At this writing, it is still unclear who carried out the devastating bombings in Madrid. The main suspects are Islamic terrorists and the Basque separatists ETA. But while identifying and finding the perpetrators is important, the bombings' most immediate lesson is that they expose anew every society's vulnerabilities to terrorism and the dangers that can lurk in something as commonplace as the morning commute.

The Spanish government's crackdown on ETA - believed to have considerably weakened the terrorist group - may call into question charges that it was behind the March 11 attacks. Moreover, within hours the Abu Hafs al-Masri Brigades, a group reputedly linked to al-Qaida, claimed responsibility for the attacks. But that group lacks credibility. It falsely claimed responsibility for last year's power outages in North America. Moreover, al-Qaida is generally less quick to claim responsibility for attacks.

Certainly the bombings differed from ETA's usual operations in their high casualty toll and lack of prior warning. Batasuna - ETA's banned political wing - issued a denial, but that cannot be taken at face value partly because the bombers may have misjudged the casualties they would inflict and the subsequent condemnation. And these attacks could as easily be done by a weak terrorist group as a strong one.

The bombings may constitute a desperate attempt by ETA or a disgruntled ETA faction to force the Basque question back onto the table. And despite the logistical difficulties involved, inflicting mass casualties is often simpler than the individual targeting ETA has previously undertaken. ETA has been well-schooled by the IRA among others, and is technically capable of such attacks, even if these dwarf their previous operations.

The bombings, just days before Spain's elections, could be an attempt to sabotage the anticipated re-election of the right-wing Popular Party. ETA has conducted pre-election attacks previously. These tactics could also result from a change in ETA's command structure after the detention of some of its key members, with new leaders deciding to risk a mass casualty attack. Last month's arrest of two suspected ETA members and seizure of more than 1,000 pounds of explosives - thought to be for an attack in Madrid - indicates that the group may have been prepared to inflict mass casualties. So does Spanish security forces' foiling of an ETA plot to blow up a train late last year.

Likewise, the disruption of ETA's plans to attack Madrid's EU-Latin American summit in May 2002 shows that ETA is prepared to expand its operations to include foreign targets. Given the attendant risks involved - especially because of the hard-line approaches to terrorism adopted by many countries since Sept. 11 - it seems plausible that ETA would be equally prepared to step up its operations inside Spain to include attacks like those on March 11.

Separately, Spain has been one of America's staunchest allies in both the war against terrorism and the decision to invade Iraq, attracting threats from al-Qaida, which is known to be active in Spain. The scale and coordination of Thursday's attacks - often cited as al-Qaida trademark - may also indicate its involvement. But any terrorist group can adopt such tactics; that is not in itself a proof of al-Qaida involvement - nor is the discovery of Arabic tapes and detonators in a suspect van. Rather, the explosives used are of a type favored by ETA, as was the method of detonation - remote control rather than al-Qaida-style suicide bombers.

The Spanish government's insistence that ETA (rather than al-Qaida, angry over Madrid's support for the U.S. invasion of Iraq) was behind the bombings is being questioned by some as a pre-election ploy. But current evidence points to ETA as likely as Islamic terrorists. The nightmarish - but not implausible - specter of a tactical alliance between the two groups is also unproved.

What the Madrid bombings do underscore is the dangers posed by terrorism's less-inventive means while targeting mundane locales. Few of us work in high-profile buildings like the World Trade Center or vacation in exotic places like Bali, but many of us go about the humdrum business of catching a rush-hour train. Improvised explosives may provide less sensational headlines than anthrax or "dirty bombs," but may prove as difficult to counter.

Panic is the wrong response to the carnage in Madrid. Instead, like the long catalogue of such incidents preceding it, Madrid should prompt sober reflection on the threat terrorism poses, even as we resolve to meet it. Terrorism will likely never be totally eradicated, but can be managed. Understanding that is the key to winning any so-called war against it.