As Americans prepare to observe the 13th anniversary of the September 11 terrorist attacks on New York and Washington, there are growing concerns that the militant group known as Islamic State could carry out attacks in the U.S. and Europe.
Each year before the anniversary of the 2001 attacks, U.S. law enforcement agencies go on alert, watching for potential terrorist attacks. Security sensitivities are higher this year because of the rise and rapid military successes in Iraq and Syria of the Islamic State, a brutal terrorist group also known as ISIS or ISIL.
The group has made a point of threatening the United States and recently beheaded two kidnapped American journalists. Terrorism experts say officials in the U.S. and other Western countries are particularly worried about hundreds of foreigners who have joined the Islamic State fighters in Syria and Iraq.
Scores of Europeans and even some Americans have known to have traveled to the Middle East to join its ranks. Terrorism analysts worry that some of these militants may return to their home countries planning to cause havoc and stage violent attacks.
Britain recently raised its "threat level" - an estimate of the risk of terrorist activity - to "severe," which is meant to signal that an attack attempt is "highly likely," but not necessarily imminent.
Home Secretary Theresa May said the threat-level adjustment was prompted by knowledge that has been gained about foreign fighters in Iraq and Syria who hold British and European passports.
“The increase in the threat level is related to developments in Syria and Iraq where terrorist groups are planning attacks against the West,” May said recently.
The U.S. Department of Homeland Security says it is unaware of any specific threats from the Islamic State. However, U.S. analysts say anniversary dates like September 11 are important for terrorist propaganda.
Two years ago, a militant attack on the September 11 anniversary razed the U.S. consulate in Benghazi, Libya, and took the lives of four Americans, including Ambassador Chris Stevens.
Global threats to the U.S. are increasing, said Professor Yonah Alexander, who directs terrorism studies at the Potomac Institute, outside the U.S. capital.
“For example, one of the affiliates of al-Qaida in Somalia, the al-Shabab, they just threatened the United States, [vowing] that they are going to mount attacks against the U.S. which are going to be worse than the U.S. experience in Iraq and Afghanistan,” he said this week.
If groups like al-Shabab or the Islamic State have willing American or European members, experts on terrorism say, those recruits can get hands-on training for attacks at home, and their militant sponsors can supply them with explosives and weapons.
Katherine Zimmerman of the American Enterprise Institute points out that militant groups such as al-Qaida in the Arabian Peninsula have been very innovative in designing undetectable bombs.
“The first bomb that the AQAP sent to the United States was concealed in the underwear of the bomber and it actually traveled throughout Europe before coming here on a plane,” Zimmerman says
U.S. authorities say they are confident in the security measures already in place. They decline to discuss any additional measures they may have taken ahead of the 9/11 anniversary.