Experts on why Manchester bomber targeted specific venue exit

Investigation into UK concert bomb

A suicide bomber killed at least 22 people and injured 59 others at an Ariana Grande concert in Manchester, England Monday night. The deadly blast took place outside of Manchester Arena after the concert, filled with young music fans, ended. Experts say the location and timing was very strategic.

"The disturbing thing about this - there were many exits. He deliberately chose that exit which was the one closest to public transportation in order to get the largest number of people," CBS News senior national security analyst Fran Townsend told  "CBS This Morning" on Tuesday.

Townsend, who is being considered for FBI director, noted that while security screening happens before an event, much of that security is gone once people leave the venue. She expects to see an effort in the future to screen people as they exit but said that crowds could still be susceptible to this type of attack.   

"Wherever we put screening, they're going to look for our vulnerabilities," Townsend said.

Townsend also called upon social media companies to do more to stop the dissemination of terrorist propaganda.

"Social media sites have to do more before these attacks to take down that propaganda, not just after them," Townsend said.

Morell says U.K. bomber was "conscious of how to maximize casualties"

CBS News senior national security contributor Michael Morell echoed Townsend's concerns about terrorists targeting the weak spots at large-scale events. He also joined "CBS This Morning" Tuesday to discuss the attack. 

Morell called the attacker's choice of location and timing, "sophisticated." 

"Whoever did this was very conscious of how to maximize casualties," he said. 

"The location was a place, as you know, where there's a large group of people exiting and the timing in terms of the end of the concert, more people flowing out massed together at the end of a concert than the beginning of a concert," Morell explained.  

Of potentially similar situations in the U.S., Morell said, "There is always a place where security ends and that's what this bomber struck and that's always possible here as well." 

For Morell, it's important to note the scale of the attack as well. 

"It's the largest attack in Europe in 10 months and the largest in the U.K. in 12 years," he said.