Tunisia attack: Why do terrorists target museums?

From the Boston Marathon bombing in 2013, to the London Subway bombing in 2005, to Nairobi shopping mall attack in 2013, and the Mumbai luxury hotel attacks in 2008, extremists have long targeted what's become known as soft targets.

In the latest example in Tunisia, militants targeted the Bardo museum, where 20 foreign tourists visiting the Tunis museum were killed by gunmen Wednesday. Security forces shot and killed the two gunmen, whose affiliation and motive is not yet clear.

On Thursday, Tunisian authorities arrested four people in connection with the attack, the first in Tunisia to kill foreigners since 2002. It is the first attack in the country’s capital, which has been struggling to attract tourists and foreign investors since its 2011 revolution.

The Tunisia attacks have already reverberated in the US, where authorities, fearful of a copycat attack, are reportedly tightening security at some of the country's most popular museums.

"My concern would be that an individual who is becoming radicalized in their basement is watching a video on the Internet of an attack and decides that they want to replicate it," John Cohen, a former counterterrorism official, told ABC News.

According to the news network, extra police have already been assigned to New York's popular museums in a direct response to the Bardo museum attack in Tunisia, and more museums may follow suit.

"I'd call this a rather loud wake-up call for major museums around the world," Peter Herdrich, vice chairman of The Antiquities Coalition, an archaeological advocacy group, told ABC.

The Bardo museum massacre is the latest in a series of terrorist attacks on cultural institutions that represents a years-long shift by militants toward soft targets.

In the years since terrorists struck airliners, the World Trade Center, and the Pentagon in the September 11, 2001 attacks, extremists have turned their focus to easier, civilian-centered sites such as hotels, shopping malls, sporting events, cruise ships – and museums.

Why the shift to soft targets?

For starters, they require significantly less planning and are harder to detect and disrupt, especially in a post-9/11 era of beefed up security.

These types of attacks are also squarely aimed at disrupting a nation's tourism and economy.

In Tunisia, where tourism accounts for 7 percent of gross domestic product, the economy relies heavily on its tourism industry, which it worked hard to regain following its 2011 revolution. Already, since the attack took place, the local stock exchange dropped nearly 2.5 percent, two German tour operators said they were canceling trips to Tunis, and Italy's Costa Cruises, a unit of Carnival Corp, said it had canceled stops in Tunisia.

And so-called soft targets like the Bardo museum also represent important symbolic, cultural treasures.

Housed in a 19th century palace next to Tunisia's parliament, the Bardo museum has been called a "jewel of Tunisian heritage," with its exhibits showcasing the country's art, culture, and history.

"It's the kind of attraction that Tunisia is promoting in an effort to diversify tourism beyond package trips to resort hotels," reported the Monitor's John Thorne, from Tunis. "For many here, it's a proud symbol of a three-thousand year heritage."

“I felt as if the Vatican or Mecca had been attacked,” Taher Ghalia, who oversaw the Bardo’s renovation and now serves as Museum's Director at Tunisia’s state heritage institute, told the Monitor, calling it “a sanctuary of culture.”