NAIROBI, Kenya (CNN) -- (CNN) -- Investigators looking into the Kenya mall attack have determined the attackers or their associates rented and operated a small store in the mall a year before the attack, according to a Kenyan intelligence official involved in the investigation.

This is an indication that the planning for the attack began more than a year ago.

Al-Shabaab, a Somalia-based terrorist group with links to al Qaeda, has said it carried out the attack.

In addition to five terrorists, at least 67 people are confirmed dead in the attack.

Officials fear that number could rise -- as many as 63 people are still unaccounted for -- as they sift through the rubble left from the collapse of three floors of the expansive facility.

Near the Ugandan border, authorities picked up three people in connection with the mall attack investigation, the Kenyan intelligence official said.

The three are among several people now being held and questioned by police over the attack Saturday at the Westgate mall.

In a news conference in Nairobi on Friday, Kenya's Interior Minister Joseph Ole Lenku said that eight suspects are being held for questioning and three others were released after interrogation. It isn't clear whether his count included the three picked up near Uganda.

CNN's Neda Fashbaf contributed to this report