Another nail in the coffin of America's malls?

It's the last thing that America's beaten down malls, already struggling with a loss of customers amid major shifts in how people shop, need -- the specter of a terrorist attack.

In an interview on CBS' "Face the Nation" Sunday, Department of Homeland Security Secretary Jeh Johnson urged people who plan to visit the Mall of America to be "particularly careful" following this weekend's threatened attack on the mammoth Minneapolis retail outlet by the Somalia-based terrorist group Al-Shabaab.

The warning left the nation's mall operators scrambling to reassure people that it is safe to shop.

"While some security measures deployed at shopping centers such as the use of bollards, uniformed patrols by law enforcement officers and surveillance camera systems will be evident to consumers, many more protective measures will not," the International Center of Shopping Center Operators, a trade group representing the country's malls, said in a statement Sunday. "Individual centers continually update and refine their security plans in concert with their law enforcement partners as part of their ongoing effort to be vigilant."

Alhough Al-Shabaab, which also threatened targets in Canada and the U.K., carried out a deadly attack on a mall in Kenya in 2013, police in Bloomington, Minnesota, where the Mall of America is located, aren't aware of credible threats against the commercial center. Still, the department has stationed additional officers there as a precaution.

Officials from the Mall of America, which accounts for nearly $2 billion a year in economic activity in Minnesota and which has its own counterterrorism unit, couldn't immediately be reached for comment.

James Forest, a terrorism expert at the University of Massachusetts Lowell, told CBS MoneyWatch that Al-Shabaab hopes its threat will inspire someone in the U.S. to carry out an attack against the Mall of America, given the logistical and security challenges of infiltrating its fighters. Many Somali immigrants have settled in Minnesota, although U.S. law enforcement officials also monitor the risks of so-called home-grown terrorists.

"Al-Shabab is really more interested in gaining attention and relevance," he said, noting that the media's attention has lately been fixed on ISIS. " There is much they can get at all from this sort of propaganda. Successful terrorists attacks require operational secrecy."

Such security concerns are flaring as the nation's malls struggle to lure back shoppers from discount chains and e-commerce platforms, which tend to undercut physical stores on price.

Howard Davidowitz, CEO of retail consulting and investment bank Davidowitz & Associates, expects as many as half of the malls in the U.S. to close down in the next 15 to 20 years.

One reason for malls' financial distress is that anchor commercial tenants such as Sears (SHLD) and J.C. Penney (JCP) have been closing stores in recent years to save money, and finding replacement stores has proved difficult. More successful retailers such as Macy's (M) and Gap (GPS) are also shrinking their store counts, as are specialty retailers including Abercrombie & Fitch (ANF).

Some communities have come up with creative solutions for what to do with abandoned malls, turning them into everything from data centers to haunted houses.

"Very few malls are opening in America -- almost none," Davidowitz said. "I get a call a day from a mall that's a wreck.... A lot of them are not viable now. We are in a crisis."

To attract customers, some malls are bringing in big-box retailers, such as Target (TGT) or Kohl's (KSS), usually found in standalone buildings. Others are featuring consumers experiences that they can't get online, such as Dave & Busters entertainment centers and medical services including massage.

Such measures are unlikely to get customers returning in droves, however.

"Will 90 percent of it fail?" Davidowitz asked. "I think it will."