Washington (CNN) Former Secretary of Homeland Security Jeh Johnson said Thursday evening that there had been no formal policy to separate the detained families of illegal immigrants under President Barack Obama.

While there were certainly "individual cases" of family separation "for reasons of health and safety," Johnson said, he denied the existence of an instituted practice during his tenure.

"It's not something I would have adopted," Johnson said on CNN's "The Lead with Jake Tapper." "It's not something I would have permitted."

Johnson explained that children were only separated from their parents should they be criminally prosecuted and placed in the custody of the Department of Justice. The decision not to prosecute every person illegally crossing the borders -- especially if he or she had children -- was deliberate for Johnson.

"I didn't believe it was right," he said. "Pulling a child away from its mother ... it's not something I could do."

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