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 .
NEW : Possible Michael Pearson Twitter account warns U.S. the attack was only Act 1 Source : Investigators link a store rented in the past year to the attackers . Despite as many as 61 in custody , there 's rising concern that some of the attackers escaped . At least 67 people are known dead ; At least 67 remain unaccounted for .