"We have reports there were up to 10 or so attackers and they appeared to be wearing a similar outfit, and others covered their faces," another police official said.
"The pattern of the attack and the way they were speaking to their targets clearly point to a well-planned attack by a terror gang," he said.
However, interior minister Joseph Ole Lenku said it was still too early to say who was responsible for the attack.
"Investigations have begun to find out the perpetrators of this crime. I urge Kenyans not to speculate," he said in a statement.
Police at the scene said a suspect wounded in the firefight had been detained and taken to hospital under armed guard.
"He appears to be a suspect and is being treated as such, he has been taken to hospital and will remain under guard for interrogation," a police officer said.
"The officers who got him out of the building have reason to believe he is one of the attackers," he said on condition he not be named.
An AFP correspondent at the scene said operations to secure the mall were ongoing.
Senior interior ministry official Mutea Iringo said "the security services will soon bring this matter of grave concern under control."