This is the latest in a string of incidents involving potential attacks on schools in Europe and the United States. The attackers and potential attackers in these cases appear to have communicated with each other on Internet forums and video-sharing Web sites.

Police ordered that Georg Buechner High School in Kaarst be closed after Finnish police discovered an Internet discussion about plans for an attack on the school. The closure came a day after police in the western German town of Cologne questioned a 17-year-old student after he had posted photos online commemorating the 1999 school shooting in Columbine, Colo. An 18-year-old accomplice was arrested after the suspect committed suicide by throwing himself in front of a tram following his interview with police. Though the accomplice was later released after police determined that he did not pose a threat, he had apparently discussed plans to use crossbows, air guns and small incendiary devices in an attack on a school.

In Norway, police questioned a suspect Nov. 19 who allegedly was planning an attack on a junior high school in the town of Askoy. The suspect in this case had posted several videos online in which he threatened an attack on the school.

Following the Nov. 7 shooting at Joleka High School in Tuusula, Finland, authorities in Europe and the United States have been on alert concerning the possibility of other attacks. In the shooting in Finland, 18-year-old student Pekka-Eric Auvinen shot and killed eight people in the school before killing himself. After the incident, authorities discovered that he had posted numerous disturbing videos on Web sites expressing a hatred for, among other things, pop culture, consumerism, religious fanatics and "the human race." In fact, Auvinen even had his own channel on the popular video sharing site YouTube under the pseudonym Naturalselector89, in which he expressed his views on Social Darwinism and clearly showed signs that he was contemplating a mass killing. In one video in which he quoted Columbine shooter Eric Harris, Auvinen made comments such as, "I will kill all. I must to be satisfied. I will not hesitate," and, "I will be famous and at the same time I will be doomed. I would have to die."

Investigators also found that Auvinen had communicated with at least one other teen who shared his views and was planning to attack a school. Several weeks before the shooting in Finland, police in Pennsylvania arrested a 14-year-old boy who admitted in court that he was planning to attack a school in a Philadelphia suburb. In a search of his home police found a rifle, air-powered guns, knives and a bomb-making manual. After the attack in Finland, the 14-year-old also confessed that he had exchanged e-mails with Auvinen in the months leading up to his arrest, though he denied that he had any knowledge that Auvinen was actually going to carry out an attack.

The fact that Finnish police alerted German authorities to the plot in Kaarst suggests that the suspects there may have communicated with Auvinen as well. Potential attackers elsewhere may have contacted Auvinen after viewing his extensive Web postings. Computer forensic experts are no doubt combing through the suspects' online activity to try to determine who else could be involved.

In past school shootings, attackers frequently communicated their intent beforehand to friends, family or potential targets. However, the difference in these cases is that there appears to be an international web of sympathizers. These incidents set a precedent for potential attackers communicating online and planning attacks together across national boundaries, similar to the "web of jihad."

This kind of international online communication represents a distinct challenge to law enforcement working to prevent school shootings. Attempting to distinguish between real threats and the countless postings by teens who sympathize with school shooters elsewhere but are not actually planning attacks requires a tremendous amount of resources. These incidents, though, demonstrate that it is an important step to take as potential attackers turn to the Internet to communicate.