Chelsea bombing: Why did the mayor wait to call it terrorism?

New York Mayor Bill de Blasio and New York Governor Andrew Cuomo tour the site of an explosion that injured 29 people in New York City's Chelsea neighborhood.

(CNN)When New York's mayor steered clear of calling a bombing in his city a terror attack, it wasn't long before critics slammed him with a common refrain.

"It's clearly terrorism, why doesn't he just come out and say it?"
But there's a good reason to be cautious about labeling violence as terrorism, says CNN national security analyst Juliette Kayyem.
"When investigators and politicians are unwilling to say it's terrorism, it's not because they're 'PC' or anything like that, it's just that the investigation has to unfold naturally," she says.
    There's a specific definition of terrorism, Kayyem says, that investigators and officials use: The purposeful attack on a civilian population for political or ideological means.
    Saturday's New York attack fits two parts of that definition; the bombing was intentional and it injured civilians.
    But there's a key element we don't know yet: The bomber's motive.
    Police captured a 28-year-old suspect on Monday. But investigators haven't released details about how they believe he's connected to the bombing.
      Even though authorities were tight-lipped about much of their investigation Monday, at least one thing had clearly changed: Mayor Bill de Blasio's approach to describing what happened.
      "Based on the information we have now," the mayor said, "we have every reason to believe that this was an act of terror."