"With that, they have to be prepared for a situation to occur when they go into large groups," Gomez said.
Here are some tips for how to stay safe at crowded public events:
Remain calm and mindful
Before joining a crowd, ask yourself: "Am I in a place where a vehicle can jump a curb and come driving at me?" Gomez said.
In the event of an attack, the "first thing you need to do is think," said Zach Hudson, an active threat expert and CEO of Grantham Systems, a Longwood, Florida-based security company that educates civilians on threat assessments and security vulnerability.
"If you're panicked, you can't think," Hudson told ABC News. "You can't mentally navigate what your immediate response should be."
To stay calm, Hudson recommended using a technique used by military and police officers: taking "three very deep breaths" while counting to four during the inhale, holding the breath for four seconds, and then counting to four during the exhale.
"That will reduce the heart rate [and] give you a couple of seconds to gather your thoughts and determine what you're going to do," Hudson said.
Avoid crowds if you can
It's important to avoid "the main rush of people that leave the venue right when the show ends," said John Matthews, executive director of the Community Safety Institute, a public safety consulting organization.
Of the dozens of people who were injured in the Manchester attack, it is unclear how many were hurt by the bomb and how many were trampled in the stampede of people rushing to exit the venue.
"Ask yourself, 'Do I really need to get the heck out of here, or can I shelter in place and await to see where the threat is really occurring and then proceed on my own in a calm manner?'" Gomez sad.
Hudson suggests "getting as low as possible" on the ground and "wait and see what's happening."
"You don't want to be one of the panicked group that everybody bolts for the exit," Hudson said. "You want to get low. You want to breathe. You want to be aware of everything around you without taking action based on fear."
If you become caught in the stampede, "look for cover" behind a steel beam, a post or a brick wall to avoid being trampled, Matthews said.
"Grab your family. Hold them close. Let that main body of people go by," he said. Then "it's safe to exit."
Watch for the choke points
A choke point is a tight space -- or blockage -- where a lot of people "have to funnel through," Hudson said. Avoiding these points of congestion, which are often exits, is crucial to staying safe during an attack, he said.
"Any choke point is going to be a high-priority point for an attacker," Hudson said, adding that it's especially important to avoid choke points in the event of a follow-up attack.
"When talking about a terrorist attack, you have to be very concerned about multiple threats," he said. If you're "running to a choke point or out of the building, you could actually be running into additional attackers."
Choke points also tend be where people get trampled on, Hudson said.
Run, hide, fight
The appropriate response a concert or moviegoer has to an active threat will be determined be his or her proximity to the actual threat, Hudson said.
Gomez advises people to remember the FBI slogan "Run. Hide. Fight."
"That is very applicable to any kind of situation, whether it is a terrorist attack, an active shooter or just some type of violence," Gomez said.
"You always want to run if you can," Gomez said.
If the threat is not immediate and you're not injured, Hudson recommends to shelter in place.
But, if there's no place to hide, "you have to defend yourself," Gomez said.
If you're close to the active shooter, then it may be prudent to attack the shooter, Hudson said. But in the event of a bomb or someone with a knife, "distance is your friend," he explained.
Awareness training is crucial for everyone
There is a "high likelihood" that someone -- a parent or an arena employee -- saw the suicide bomber beforehand and "didn't think anything of it," Hudson said.
"It's not like he beamed into place," Hudson said. "More than likely, he was there for a while."
The attacker probably surveyed the area beforehand and "decided where he was going to set up," Hudson said.
Moreover, anyone who saw the attacker likely had no security assessment training, and therefore wouldn't have the wherewithal to say something or call 911, he pointed out.
"Awareness becomes a key facet to the safety of your kids," Hudson said.
Hudson gave the example of someone wearing a jacket in the middle of a scorching summer day. If the person is wearing a jacket and is hanging out in a choke point, those are two "flags." And if the person appears to have a "thin face" and "thin legs" but is sporting a big belly under the jacket, more red flags are raised.
Another warning sign: if you see someone place a backpack inside of a garbage can near an exit.
"That's a 911 call," he said. "Your average everyday person doesn't look for those things."
The timing of the Manchester attack may shed light on a new terror trend, experts say
At the Manchester Arena, the attacker chose to detonate the bomb at the end of the evening, when people were "tired," "ready to go home" and security was more lax, Gomez said.
"There's less of a chance that people are alert for someone suspicious such as a terrorist with a bomb," he said.
Gomez compared the timing of the Manchester attack to last summer's shooting rampage at the Pulse nightclub in Orlando, when shooter Omar Mateen chose the end of the night -- minutes before the club closed -- to begin opening fire.
"This could be a trend that terrorists are now going to follow with regard to when they commit their attack," Gomez said, especially since screening before a high-profile event tends to be extensive.
Where the bomb was detonated -- just outside the arena -- highlights the "soft target" at a venue or even an airport in the area prior to security screenings, such as the attack at the Brussels airport last year.
"We have seen this for several years at the airports, where the area leading up to the airport screening is a vulnerability," Gomez said. "Security officials have to take that into account."
Much of the burden to ensure safety falls on the venue
There has to be a security vulnerability assessment for the venue, Hudson said, adding that several organizations don't invest enough in safety and security. In the event of an attack, it is a "systems issue" to control the exits for a lockdown, Hudson said.
In addition, it's important for security personnel to have had awareness training, Hudson said.
"You want, as an owner of a venue, to make sure you're projecting a security presence beyond the wall," he said.
For parents, it would be "worth inquiring" with the event staff on what kind of security they're going to have either at the event or outside of it, he added.