Officials say publicly there is no “credible threat” of an attack on America’s shopping malls, despite an on-line “call to action” by terror group al Shabab -– but behind the scenes authorities are taking the threat seriously, because previous calls to action have been heeded by violent “lone wolves.”

Last October a soldier was shot and killed in Ottawa just steps from Parliament in what Canadian officials called a terrorist act. The FBI says a bloody hatchet attack on NYPD officers the same month was also terrorism. Both attacks came after ISIS urged lone offenders in the West to attack “soldiers, patrons and troops” and members of law enforcement.

“Calls to action are a way a terrorist group like al Shabab can project itself without expending resources,” said a U.S. counterterrorism official. “It’s a way to awaken and embolden those who may not be in contact with the group but share their values. And unless there’s some other information out there … they are in our blind spot.”

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Al Shabab, a Somalia-based Islamist terror group that killed nearly 70 people in a 2013 attack on a Kenyan mall, released an on-line video Saturday that called for attacks on Western malls. It singled out the West Edmonton Mall in Canada, London’s Oxford Street shopping district and the massive Mall of America in Bloomington, Minnesota.

Michael Sheehan, chair of the Combating Terrorism Center at West Point and an NBC News analyst, said that attacks on malls had been predicted "since the day after 9/11, but this is the first time I know of it was specifically called for."

Security was enhanced at the Minnesota mall, which already has a permanent police presence.

“We’ve been hearing of the potential for lone wolf attacks for some time,” said a senior law enforcement official in the Minneapolis/St. Paul area. The official said the risk for lone wolf attacks has led to “ramped-up active shooter training” and drills in recent years. Local agencies have trained for active shooter situations in large public places like malls.

Al Shabab’s potential threat to the Twin Cities is also not new. Minnesota has one of the largest Somali immigrant populations in North America. At least 20 local Somali-American men have traveled to Somalia since 2007 to train and fight with al Shabab.

Three U.S. citizens have been involved in suicide attacks in Somalia, and at one time Shabab had as many as 50 different U.S. citizens fighting in its ranks. Another 200 al Shabab fighters have passports that would permit them to enter the U.S. without a visa, like tourists.

Al Shabab’s attack on Nairobi’s Westgate mall left 67 people dead in September 2013. Though the group has had military setbacks in recent years, it’s still capable of mounting large-scale terror operations. This past weekend, militants assaulted a luxury hotel in Mogadishu and killed 25 people, including top Somali politicians.

In Minneapolis, authorities are also investigating a handful of individuals who may have traveled to Syria to fight with ISIS, and may pose a risk of becoming lone wolf attackers if they return to the U.S.

“It is not a matter of if, but when” such an attack will occur somewhere in the U.S., said the Minneapolis-area official.