Transcript for How do you protect 'soft targets?'

tonight after the NBA finals. We want to bring in former Dallas police chief and ABC news contributor, David brown. And Matt Olson in Washington. Chief brown, let's beginning with you. The concert in Manchester will go on. How you to secure an ept like this pr a police perspective just hours after this attack? The most important work that should occur is the intelligence. We need to learn if there were accomplices. We know we have the three suspects dead. Did they plan other attacks? Did they plan attacks at this concert? Next, you see a big presence of police. Particularly at entrance points. And it's also important to have a big presence at exit points when the concert is over. That's a very vulnerable time for attacks in the crowd. No question. We're heading into a stressful few hours for law enforcement in the uk. Matt, we keep hearing about and seeing attacks on what's called soft targets. What can we do to protect soft targets? You know this is the challenge. For security officials and police departments across Europe and here in the U.S. It's very difficult if not impossible to protect all soft targets. Pedestrian walkways, restaurants, bars. I agree with the chief. Good intelligence is the key. Understanding the sbepgss and capabilities of individuals at the earliest possible stage so they never get to the point of carrying out an take. Do we have I love with a certain amount omt vulnerability? Yes, I think so. And I think people have to be engaged more to be a part of the solution. To their public safety. Citizens when they see something like we say in the united States, say something. Engage. Be involved. Anything looking out of place, immediately notify authorities. It makes a job for enforcement officials and police so much more difficult because the weapons of choice are vehicles and knives. Real quick, Matt, you say if this attack happened in the U.S., it could have been much worse? The concern is access to guns. Particularly assault weapons. Here we have seen attacks with knives and a car. Of course, with a gun, this could have been much worse. Much more lethal. Matt Olson, chief brown, thank you both so much. Thank you. Let's switch gears and check

This transcript has been automatically generated and may not be 100% accurate.