A terror attack unfolded Thursday when a van mowed down pedestrians in Barcelona's popular Las Ramblas district. At least 13 people were killed and scores of people injured. The Islamic State of Iraq and Syria (ISIS) claimed responsibility, although CBS News hasn't seen evidence to support that claim yet.

Officials say two suspects have been arrested in connection with the attack. Police say one suspect is a Spanish national from Melilla and the other is Moroccan. Neither of the suspects drove the van, and police said the driver is still on the run.

Fran Townsend, a former homeland security adviser to President George W. Bush and CBS News senior national security analyst, spoke with Anthony Mason on "CBS Evening News" about the attack and the terrorism threat.

She says that the latest attack is the enemy taking advantage of targets where security may not always be in mass quantity.

"Over the years, governments around the world have put up barriers in large pedestrian areas to prevent cars from getting into them, but you can't do that everywhere," Townsend said. "Any iconic, tourist landmark -- any place people gather, restaurants, squares -- we can't protect all of them. Sadly, the enemy, ISIS, is taking advantage of them."

Townsend points out that in civilian populations, ISIS has a terrorist advantage since security can't be everywhere at all times.

"We shouldn't be all that surprised," she adds. "We are succeeding against them in place like Iraq and Syria. They clearly are looking for opportunities to strike back."

"We'll see if this is foreign fighters which are far much more prevalent in places like France, Germany and Brussels," Townsend said.

She also noted that it's difficult to prevent attacks like this -- especially in European nations.

"We know that in the United States, the FBI gets these sorts of leads all the time -- it's almost overwhelming, they don't have enough resources. So they make judgments as to who they will follow," Townsend said.

Meanwhile, three days of mourning have been declared in response to the attack. A minute of silence will be held Friday in main square "to show that we are not scared," Barcelona's mayor, Ada Colau, said.