With bomb-sniffing dogs, bag inspections and rows of metal detectors at the entrance, the modern concert arena is in some ways a fortress.

But the blast that killed 22 people on Monday at an Ariana Grande concert in Manchester, England, highlighted the dangers that still exist along the perimeters of these buildings — on the street or in public concourses where concertgoers and others may gather in large numbers, unexamined by any security force.

Investigators say the explosion at Manchester Arena occurred in a foyer just outside the venue’s doors, a space that connects the arena to the nearby Victoria rail station. SMG, the company that manages the arena, said that it is not responsible for policing that space.

The episode immediately recalled the attacks in Paris in November 2015, when gunmen who entered the Bataclan theater during a performance killed 90 people. But Steven A. Adelman, the vice president of the Event Safety Alliance, a trade group, believes that comparison is not quite apt.