"You don't bring something like a crew-served weapon through the door," an American official said. "Those must have been stored well beforehand."
At least 61 civilians and six Kenyan security officers died in the four-day attack and rescue efforts, said President Uhuru Kenyatta.
Officials said al-Shabab militants hid their guns in one of the mall's stores days before the attack, with an employee's help.
At least one also stored extra clothes to change after the shooting, abandoning their old clothes and the rifles in the mall.
The attack was "rehearsed," and details as minute as the building's ventilation system and locations of service personnel stairways were studied.
The terrorists were neither "interested in hostage-taking" or negotiating, a Kenyan official said.
"They only wanted to kill." The terrorists did allow some Muslims to leave the mall after interviewing them at gunpoint to confirm their religion, survivors said.
Meanwhile, shots were still ringing out at the mall this morning, despite the fact that Kenya's president said yesterday the siege was over. An al-Shabab Twitter account said the Kenyan government purposefully demolished the mall, despite reports that the mall collapsed due to a militant-set fire.