Nairobi, Kenya (CNN) -- To anyone shopping at Nairobi's Westgate Mall, it would likely have seemed just another store.
But according to a Kenyan intelligence official, the small shop concealed an ominous secret. It was rented by the Al-Shabaab terrorists, or their associates, who within a year would carry out an attack on the upscale shopping mall.
The information -- revealed Friday to CNN by the source, who is close to the investigation into the attack -- suggests the Somalian terror organization had been planning the operation at least that long.
How the team of terrorists got their weapons and explosives into the mall without notice is a central part of the investigation into the attack, which left at least 67 people dead and parts of the upscale mall in ruins.
The Kenya Red Cross said Friday that 61 people remain unaccounted for. Some could be buried in the rubble of the partially collapsed mall.
At least five of the terrorists also died before Kenyan forces were finally able to bring the siege to an end on Tuesday. The terrorists stormed the building Saturday.
On a Twitter account believed to be run by Al-Shabaab, the group promised more attacks to come.
"The mesmeric performance by the #Westgate Warriors was undoubtedly gripping, but despair not folks, that was just the premiere of Act 1," according to a tweet posted Thursday.
CNN could not confirm the authenticity of the tweet, but CNN terrorism analyst Paul Cruickshank said the account, which has also posted links to statements from Shabaab leader Mukhtar Abu Zubayr, appears to be legitimate, even if not "100% authenticated."
Several Twitter accounts attributed to Al-Shabaab have been shut down in recent days, likely for violating the company's rules against promoting violence in tweets.
While Kenyan Interior Minister Joseph Ole Lenku said Friday that eight suspects are being held for questioning in the attack, authorities are increasingly concerned that some of the attackers managed to escape alongside fleeing civilians in the aftermath of the initial attack, U.S. law enforcement officials told CNN.
On Thursday, a Kenyan counterterrorism source told CNN that one of the suspects is an injured Kenyan who was being evacuated when a machine gun magazine fell out of his pocket, leading to suspicion he was among the automatic-weapon toting terrorists who roamed the mall killing civilians. He is being held in a military hospital, the source said.
Among the suspects are three people picked up near the Ugandan border, the Kenyan official who revealed information about the mall store told CNN.
CNN's Michael Pearson reported and wrote from Atlanta; Nima Elbagir reported from Nairobi; CNN's Neda Fashbaf also contributed to this report