(CNN) -- It's hard to imagine a softer target than an enclosed, easy-to-enter space with large numbers of civilians, many of them children or elderly, milling about with no authority clearly in charge.
If you connect those dots, you get the kind of scenario that "keeps us up at night," as a federal law enforcement source told CNN's Pamela Brown: an attack at a shopping mall in the United States.
Can it happen here? Yes, say security experts, but it hasn't.
There have been shootings at U.S. malls, although not by terrorists. And there have been terror plots that were foiled.
But the United States has not witnessed anything close to the scope of the violent siege at Nairobi's Westgate mall.
There, the standoff continued Monday morning, two days after Al-Shabaab gunmen barged in and sprayed gunfire, killing at least 68 people and injuring 175.
Three of the alleged attackers lived in the United States. According to sources within Al-Shabaab, two lived in Minnesota and one in Missouri. And they are all in their 20s.
A senior U.S. State Department official says they're still trying to match the names, but they're becoming more confident that American citizens may have been involved.
It would not take too much sophistication to try a similar attack here, the official who spoke to Pamela Brown said.
"The worst case scenario is a bunch of these kids coming back, buying weapons in the United States some place like Minneapolis or Chicago and going after one of our malls here," Bob Baer, CNN's national security analyst, said Sunday. "They are indefensible especially with a well-trained group. There's nothing you can do about it. And I guarantee you that the FBI is going to be on it today."
In the past few years, federal prosecutors say they have thwarted two planned attacks on malls, each of which would have been carried out by single attacker:
--Nuradin M. Abdi, a Somali citizen living in Columbus, Ohio, was sentenced in 2007 to 10 years in prison after admitting he sought terrorist training in Ethiopia to carry out attacks, including a never-attempted attack on a mall in 2002. --Derrick Shareef of Rockford, Illinois, was sentenced in 2008 to 35 years in prison after pleading guilty to plotting to set off grenades at a Rockford shopping mall. Shareef was a convert to Islam who was recorded saying he wanted to kill "infidels."
But attacks which have succeeded in causing casualties at American malls in recent years have been carried out by young lone gunmen with no apparent cause to promote:
--A 19-year-old man killed eight people and then himself at an Omaha, Nebraska, mall in December 2007. --An 18-year-old man killed five people before he was killed by police at a mall in Salt Lake City, Utah, in February 2007. --A 22-year-old man killed two people and then himself at a mall near Portland, Oregon, in December 2012.
"Soft targets always attract the terrorists because they're usually not defended," said Lt. Col. Rick Francona, CNN's military analyst. "It's a very effective way of causing a lot of panic, a lot of damage very quickly and achieving the objective of terrorizing people."
At the Westgate Mall in Nairobi, Kenya, multiple attackers went from store to store killing shoppers. The attackers then took hostages to stymie arriving authorities. The terrorists' apparent plan was not to negotiate but simply to prolong the standoff and get more media coverage, CNN security analyst Peter Bergen said.
"I think that if you're looking for a hundred percent safety, you should probably wrap yourself in bubble wrap and never leave home," said Doug Reynolds, security director of the Mall of America.
A strategy to minimize the damage a lone attacker or an armed group could do before authorities arrive can be seen twice a month at the giant mall in Bloomington, Minnesota, which is visited by 43 million people a year.
A voice comes over the public address system and announces that everyone, customers included, should take shelter in back rooms of the mall's stores. Employees lock doors and lower security gates.
"If something bad should happen here, we don't want our response to start with, 'and law enforcement will be here and will protect you,'" Reynolds said. "We want to know what can be done before law enforcement gets here."
Preventing a return
The mall is in the suburbs of Minneapolis, home to a Somali-American community where some families have lamented the loss of young men who were enticed to Somalia to fight for Al-Shabaab, the group responsible for the Kenya attack.
The House Committee on Homeland Security reported in 2011 that an estimated 40 Americans have joined Al-Shabaab in the last few years, including 24 from Minneapolis.
And so, federal law enforcement are working with the Somali-American community in Minneapolis to try to prevent other Al-Shabaab recruits from returning to the United States and committing similar acts.
In the end, however, there is only so much precaution one can take, said Tom Fuentes, a law enforcement analyst for CNN.
"For the average American citizen, you go to the grocery store, you go to the gas station, you go to the shopping mall, and you go to a movie theater. You take walks in your neighborhood," he said.
"Anyone of those situations could make you vulnerable if other people or another person is out there determined to conduct an attack."