Although the total number of terrorist incidents worldwide declined during the 1990s, the number of people killed in terrorist incidents increased. Thus, while terrorists may have become less active, they also became alarmingly more lethal. One key factor behind this trend is the amount of terrorism motivated by religious views, as were the attacks on New York City's World Trade Center and the Pentagon in Arlington, Virginia, on September 11, 2001 (see September 11 Attacks). Terrorism motivated by religion has frequently led to acts of violence with higher levels of fatalities than the relatively more targeted incidents of violence perpetrated by many secular (nonreligious) terrorist organizations.

Another key factor that has contributed to terrorism's rising deadliness is the ease of access to a range of low-tech and high-tech weapons. At the low-end of the weapons spectrum, terrorists rely on guns and bombs, as they have for more than a century. At the high end of the spectrum, there is evidence that groups such as al-Qaeda seek to acquire chemical, biological, and even nuclear weapons (see Chemical and Biological Warfare; Nuclear Weapons). Other terrorist groups, such as Aum Shinrikyo, already have carried out terrorist attacks using biological and chemical weapons. It is feared that the nuclear weapons stockpiles of the former Soviet Union could produce an international black market in fissionable materials that terrorists might potentially obtain. Finally, in the middle range of the weapons spectrum the world is awash in sophisticated items available to terrorists everywhere, including plastic explosives and hand-held, precision-guided surface-to-air missiles (SAMs).

An increase of suicide attacks has also contributed to terrorism's rising death count. Suicide attacks differ from other terrorist operations, because the perpetrator's own death is a requirement for the attack's success. Suicide bombers, therefore, are typically highly motivated, passionately dedicated individuals who decide voluntarily or upon persuasion to surrender their lives in fulfillment of their mission.

A wave of suicide attacks began in 1981 in Beirut, Lebanon, when a group called al-Dawa used a car bomb to blow up the Iraqi Embassy. Al-Dawa, (“the call” in Arabic, as in “the call for Holy War”) was a terrorist organization composed of Shia Muslims from Iraq who were backed by Iran. (Muslims belonging to the Shia branch of Islam form a minority in Iraq but the majority in Iran.) The Beirut attack killed 61 people and wounded more than 100 others. In 1983 a truck filled with explosives drove into the U.S. embassy in Beirut, killing 49 and wounding 120 others. It was followed later that year by a suicide bombing of U.S. Marine barracks in Beirut, which killed 241 persons. A group called Islamic Jihad claimed responsibility for both attacks. Another suicide bombing destroyed a Jewish community center in Buenos Aires, Argentina, in 1994, killing 96 persons. More recently, al-Qaeda staged suicide attacks on the United States embassies in Kenya and Tanzania in 1998, causing nearly 300 deaths; on a U.S. Navy warship the U.S.S. Cole in 2000, causing 19 deaths; and on the World Trade Center and Pentagon in 2001, causing about 3,000 deaths. Many of the attacks carried out by Palestinian organizations, such as Hamas and Islamic Jihad, in Israel and the Occupied Territories have involved suicide bombings. Other terrorist groups also have adopted this tactic, including Tamil separatists fighting in Sri Lanka and India, and Kurdish separatists in Turkey. These separatists belong to ethnic minorities that seek to set up separate homelands.

Terrorists today claim credit less frequently for their attacks than they once did, a fact that also reflects terrorism's increasing deadliness. Unlike today's reticent terrorists, the more traditional terrorist groups of the 1970s and 1980s not only issued communiqués explaining why they perpetrated an attack but also boasted proudly after a particularly destructive or deadly operation. The current trend toward less communication implies that violence may be less a means to an end than an end in itself for some terrorist groups. In other words, terrorists today may use violence simply as vengeance or punishment rather than as a means to achieve political change. Therefore, their actions require no explanation or justification outside the terrorist group itself or its supporters.