We are still waiting to learn who perpetrated the Boston Marathon bombings, but it would not be surprising to find out that the attack was the work of a lone-wolf terrorist. Lone-wolf attacks have become more common as individuals have gained the ability to strike without the training and funding traditionally provided by terrorist groups. All they need is access to the Internet. Although statistics on the number of lone-wolf attacks vary due to discrepancies in definitions and data collection, government officials and law enforcement agencies in many countries agree that the threat is growing. Recent lone-wolf attacks have spanned the political and religious spectrum. They include an anti-Islamic lone wolf, Anders Breivik, who set off a car bomb in Oslo and then traveled to an island to massacre scores of youths attending a summer camp, killing 77 people in all. In the United States, an Islamic extremist, Maj. Nidal Maljik Hasan, is accused of shooting fellow soldiers and others at Fort Hood, Texas. There have also been recent lone-wolf plots and attacks by neo-Nazis, white supremacists, and "single-issue" extremists. What makes lone wolves so dangerous is their ability to think outside the box. Since they operate by themselves, there is no group pressure or decision-making process that might stifle creativity. Lone wolves are free to act upon any scenario they can dream up. This freedom has resulted in some of the most imaginative terrorist attacks in history. For example, lone wolves were responsible for the first vehicle bombing (1920), major midair plane bombing (1955), hijacking (1961), and product tampering (1982), as well as the anthrax letter attacks in the United States (2001). Lone wolves also have little or no constraints on their level of violence. Because they are not part of a group, lone wolves are not concerned with alienating supporters (as many terrorist groups are), nor are they concerned with a potential government and law-enforcement crackdown following an attack. Lone wolves are also difficult to identify and capture. Because they work alone, there are usually no communications to intercept or co-conspirators to arrest and interrogate. That is the reason why Theodore Kaczynski, the infamous "Unabomber," was able to send package bombs throughout the United States for nearly 17 years. The current wave of lone-wolf attacks has been propelled by the revolutionary impact of the Internet, which provides lone wolves with limitless opportunities. Online, an aspiring terrorist can find everything from instructions on building homemade bombs to maps and diagrams of potential targets. Detailed accounts of terrorist incidents around the world are also readily available, providing guidance and perhaps inspiration. In addition, websites, blogs, Facebook pages, and chat rooms all provide easy venues for cultivating extremism in a way that was previously possible only through in-person gatherings. Who, then, are these lone wolves? There are five basic types -- secular, religious, single-issue, criminal, and idiosyncratic -- although some lone wolves fall into more than one category. Secular lone wolves, like secular terrorist groups, commit violent attacks for political, ethnic-nationalist, or separatist causes. Timothy McVeigh, who killed 168 people at the Alfred P. Murrah Federal Building in Oklahoma City in 1995, was motivated by revenge for the U.S. government raid on the Branch Davidian cult's compound in Waco, Texas, which had killed 80 people two years earlier. Terry Nichols helped build the bomb, but McVeigh acted largely alone. Although Anders Breivik is certainly anti-Muslim, he also falls into the secular category because his main targets were not Muslims, but rather the teens attending a camp run by the ruling Labor Party, which he blamed for allowing the "Islamic colonisation and Islamisation of Western Europe." The second type of lone-wolf terrorist is the religious lone wolf, who perpetrates terrorism in the name of Islam, Christianity, Judaism, or some other belief system. Major Hasan falls into that category, having been influenced in part through numerous e-mails he exchanged with Anwar al-Awlaki, the American-born Islamic extremist cleric who was living in Yemen before he was killed by a U.S. drone strike in 2011. The third type of lone-wolf terrorist is the single-issue lone wolf, who perpetrates attacks in the name of some specific issue, such as abortion, animal rights, or the environment. Among the single-issue lone wolves is Eric Rudolph, the anti-abortion militant who set off a bomb at the 1996 Summer Olympic Games in Atlanta and then bombed abortion clinics. On two occasions Rudolph left two bombs at the scene of his attack, demonstrating that it is possible for a lone wolf to perpetrate multiple bombings at one site, as occurred at the Boston Marathon. Volkert van der Graaf, an animal-rights extremist who assassinated a Dutch politician in favor of legalizing mink farming, is another example. While secular, religious, and single-issue lone wolves have, in many ways, the same objectives and motivations as terrorist organizations, the fourth type of lone wolf is more distinctive. The criminal lone wolf is motivated mainly by money and personal gain, as opposed to political, social, religious, or ethnic-nationalist goals. The lack of an ideological objective is why most analysts do not consider lone-wolf criminals to be "terrorists." However, there are some cases where criminal activity has so great an effect on society and government that it should be considered terrorism. For example, the first major midair plane bombing in U.S. history occurred in 1955, when 23-year-old John Gilbert Graham put several sticks of dynamite and a timer in his mother's luggage before she boarded a United Airlines flight out of Denver. Graham was hoping to collect a $37,500 insurance policy on her life. Forty-four people, including Graham's mother, were killed. The bombing shocked the nation and led the FBI and the Civil Aviation Administration to study ways to detect explosives in luggage. One alleged criminal lone-wolf terrorist was Bruce Ivins, an Army microbiologist who the Justice Department, FBI, and the U.S. Postal Inspection Service concluded was responsible for the 2001 anthrax attacks. The Justice Department speculated that Ivins, who committed suicide in 2009, hoped to revitalize his research on anthrax vaccines. The attacks, which shut down government buildings and mail-processing centers, created a crisis atmosphere in the United States concerning the threat of bioterrorism. The fifth type of lone-wolf terrorist is the idiosyncratic lone wolf, who is motivated largely by personal demons. This category of lone wolves is also unique, since, with the exception of cults that commit terrorist acts, there are really no idiosyncratic terrorist groups in operation. Although the idiosyncratic lone wolf may commit attacks in the name of some cause, those causes are often irrational, and the perpetrators are driven to violence chiefly by severe personality and psychological problems. Theodore Kaczynski called for a revolution against the industrial-technological society, but he was a paranoid schizophrenic. Muharem Kurbegovic, also known as the Alphabet Bomber, who set off a bomb at Los Angeles International Airport in 1974 and demanded that all immigration, naturalization, and sex laws be declared unconstitutional, was also a paranoid schizophrenic. The same is true of Jared Loughner, who killed several people in a failed attempt to assassinate Arizona Congresswoman Gabrielle Giffords in 2011. The Internet has proven indispensible to every type of lone wolf. Volkert van der Graaf, the Dutch animal rights activist, searched the Internet for information on his target's daily schedule and obtained a map of the Dutch town where he carried out the assassination in 2002. Colleen LaRose, also known as "Jihad Jane," used MySpace, YouTube, and e-mail to contact other extremists and express her desire to become a martyr for the Islamic cause. In August 2009, she traveled to Europe to take part in an assassination plot against Lars Vilks, a Swedish illustrator who had angered Muslims throughout the world with his derogatory caricature of the Prophet Muhammad. One particularly remarkable case is that of Roshonara Choudhry, who was an excellent student at the prestigious King's College in London before unexpectedly dropping out just a few months before graduation in 2010. She then attempted to assassinate Stephen Timms, a British member of parliament who had supported the Iraq war. Choudhry showed no signs of radicalization prior to the attack and gave no indication to friends, family members, or acquaintances that she sympathized in any way with Islamic extremism. But for months she had been secretly downloading sermons by Anwar al-Awlaki -- more than 100 in all. She never met, e-mailed, or talked to al-Awlaki, but was motivated solely by his online calls for violence against the West. As valuable as the Internet is for lone wolves, it can also cause them problems. Lone wolves become most vulnerable to discovery by law enforcement whey they surface online. Whether it is by announcing to the world, as LaRose did, that she was ready, willing, and able to commit terrorist acts, or by posting manifestos as Breivik did, lone-wolf terrorists can leave many Internet clues. The chat rooms they frequent and the online searches they conduct can all be potential pitfalls. In fact, had the Internet been accessible during Kaczynski's reign of terror, he probably would have been caught a lot earlier. Kaczynski desperately sought an outlet for the dissemination of his anti-technology, anti-industrial-society views. That is why he demanded that newspapers publish his manifesto. He very likely would have posted his writings online early in his terrorist career (despite his distaste for technology). The scenario that played out years later -- his brother turned him in after reading the manifesto in a newspaper -- would have likely occurred once his brother read it online. Subsequent attacks may have therefore been prevented. Another lesson learned from the study of lone-wolf terrorists is that lone wolves are not as crazy as many people assume. While some are mentally ill, such as Kaczynski and Kurbegovic, and others have psychological problems, such as Breivik, many are not "abnormal' in the psychological sense, such as Rudolph and Choudhry. While some lone wolves combine personal grievances and problems with a political or religious cause to justify their violence, many others are as dedicated to the issues for which they are fighting as are "regular" terrorists. It is also a myth that little can be done to prevent lone-wolf terrorism. In addition to monitoring the Internet, other measures can be taken to reduce the risk of lone-wolf attacks. These include the continual development of devices to identify package bombs or letters containing anthrax spores; the expansion of closed-circuit television cameras in public settings; and further advances in biometrics, including the use of gait analysis, which can determine if a person is carrying a bomb, and facial-expression analysis, which can predict hostile intent. In addition, there is the obvious, but usually ignored, public responsibility to report unattended packages at airports, bus terminals, shopping malls and other possible targets. We don't yet know how long the bags containing the bombs were left unattended at the Boston Marathon's finish line, but there might have been enough time for somebody to notify the police and begin moving people away from danger. For too long, scholars, experts, government officials, and the public have focused on terrorist organizations, such as al Qaeda, to the exclusion of the individual. Lone wolves have proven repeatedly that they can be as dangerous and have as much impact on societies and governments as the larger, better-financed, and better-trained terrorist organizations. Lone wolves have become important players in the world of terrorism and will have to be reckoned with now and for the foreseeable future.