"Anytime a terrorist organization calls for an attack on a specific place, we've got to take that seriously," Johnson said.
He also called on the public to play a role in helping safeguard possible attack locations.
"We've seen this a number of times now where a group will call for an attack on a country, on a specific location," Johnson said on Face the Nation. "So, we've got to be vigilant."
"I am not telling people to not go to the mall,” he said. “I think there needs to be an awareness. There needs to be vigilance. We're in a new phase right now, and that involves public participation in our efforts."
Al Shabaab is believed to be responsible for a high-profile attack on a high-end Westgate shopping mall in Nairobi, Kenya in 2013 during which the ykilled 67 people. Mall, some analysts say, make good targets because they are typically lacking in security and full of unaware shoppers.