Is a Terrorist Attack More or Less Likely on 9/11?

New York City Police Champlain Khalid Laitif stands at the edge of the North Pool during memorial observances on the 13th anniversary of the 911 attacks at the site of the World Trade Center in New York Robert Sabo/POOL/Reuters

As the 13th anniversary of the most deadly terrorist attack on American soil approaches, security agencies in the United States and around the world are being called on to raise alert levels. An imminent attack to commemorate the occasion is much feared.

There are, needless to say, no September 11 references in the Koran. Muslim holidays are marked according to a lunar calendar, in which September may coincide with different religious events on any given year.

Terrorist groups of the extreme-right variety often strike around April 20, the day of Adolf Hitler's birth in 1889. For that reason, the April 15, 2013, Boston Marathon attack was at first thought by some investigators to be the handiwork of a domestic right-wing group. It later was found to be the work of two brothers, Chechen Islamists, who had no religious-based reason to pick the April date.

On the other hand, some history buffs have tried to link the decision to strike New York and Washington on September 11, 2001, to the 17th century Battle of Vienna. The Islamic Ottoman Empire suffered a humiliating defeat in that battle on September 11, 1683, losing to a coalition of German, Pole and Lithuanian Christians in what some historians consider a turning point in the 300-year war between the Ottomans and the Holy Roman Empire.

While that theory is supported by no hard data, there is little doubt that since the 2001 attacks, September 11 has become a time for heightened alerts on both ends of the terror equation.

Additional anti-terrorist measures are enacted every year in world capitals. Reports of jihadists plotting new attacks become more frequent and are taken more seriously than at other times of year. And as demonstrated in the 2012 Benghazi, Libya, attack, Islamists have managed to successfully strike on or around September 11.

The jihadists "take anniversaries very seriously in terms of choosing when to attack in the United States," Representative Michael McCaul, R-Texas, chairman of the House Committee on Homeland Security, told CNN last week. "I think we need to be on a high state of alert."

He spoke shortly after British Prime Minister David Cameron raised his country's terror threat level from "substantial" to "severe," the second highest alert level.

Also at the end of August, King Abdullah of Saudi Arabia predicted at a Riyadh reception for foreign ambassadors that unless rising jihadi groups like the Islamist State (best known as ISIS, and sometimes called ISIL) are confronted militarily, they may strike Europe "in a month" and hit America a month after that.

More concretely, press stories predict a multi-plane strike attempt on September 11, following the reported theft in late August of 11 commercial jetliners by jihadi groups from the Tripoli, Libya, airport.

Washington officials decline to comment, or say they have no information on the missing planes. But the Tripoli airport was bombarded by the air forces of Egypt and the United Arab Emirates in late August, indicating a growing concern among Libya's neighbors about the lawlessness in the country—and the rise of several jihadi groups there.

Such reports have cropped up often since 2001, in the time between Labor Day and the end of September, but fears may be more justified now than in the past. The geopolitical terrorist map is changing as groups like ISIS make gains in the Middle East and as they recruit Westerners, who can more easily travel to Europe and America.

Additionally, while a United Nations team has completed its work on the destruction of Syria's chemical weapons, there is suspicion that some undeclared stockpiles remain.

"Certainly if there are chemical weapons left in Syria, there will be a risk that those weapons fall into ISIL's hands," Samantha Power, America's U.N. ambassador, told reporters recently. She added, "We can only imagine what a group like that would do if in possession of such a weapon."

But is the significance of the September 11 date growing among these newcomers or among older jihadists who may feel overshadowed and therefore become more dangerous?

In the early years after 2001, fears of attacks on or around September 11 failed to materialize, says Hedieh Mirahmadi, president of the World Organization for Resource Development and Education, an organization promoting understanding between Muslims and non-Muslims. "But in the past couple of years, I would say yes," she said.

The Madrid train bombings in 2004 took place on March 11, while the London underground bombings in 2005 were on July 7.

On September 11, 2012, the American diplomatic compound in Benghazi was attacked by a group led by Ahmed Abu Khattala, the leader of Libya's jihadist militia Ansar al-Sharia. He was captured this past June by U.S. commandos.

Last year, members of the Somalia-based Islamist group Al-Shabab attacked the Westgate shopping mall in Nairobi, Kenya, where at least 67 people were killed. In claiming responsibility for the attack, the group cited Kenya's participation in combat against it. And while the three-day attack started on September 21, it may well have been planned for the month of the 2001 attacks.

Such operations are not planned with perfect precision so as to be carried out exactly on September 11, says Mirahmadi, who specializes in transnational Salafi movements. But, she says, "9/11 has this horrific significance for global jihadists; it marks a turning point" in their battle against America.

The Koran, she adds, marks dates in which fighting is specifically forbidden, as in the times around the Hajj, a pilgrimage to Mecca. But there are no calendar dates designated specifically for waging battle.

But jihadists have long bent religious edicts to justify acts that have little to do with Islam. And even if September 11 has gained near-religious significance, they may well choose to strike on another date, or at completely different times of year. After all, opportunity plays a much bigger role in planning terror attacks than holy dates.