Story highlights Possible Al-Shabaab Twitter account warns Nairobi the attack was only "Act 1"

Source: Investigators link a store rented in the past year to the attackers

Despite 8 in custody, there's rising concern that some of the attackers escaped

At least 67 people are known dead; as many as 61 remain unaccounted for

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.

Photos: Photos: Kenya mall attack Photos: Photos: Kenya mall attack Kenya mall attack – Relatives of Johnny Mutinda Musango, 48, weep after identifying his body at the city morgue in Nairobi, Kenya, on Tuesday, September 24. Musango was one of the victims of the Westgate Mall hostage siege. Kenyan security forces were still combing the mall on the fourth day of the siege by al Qaeda-linked terrorists. Hide Caption 1 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – Ann Gakii reacts at the Nairobi City Mortuary after identifying the body of her father, who was killed in the mall attack on Saturday. Hide Caption 2 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – A Kenyan soldier runs through a corridor on an upper floor at the Westgate Shopping Mall in Nairobi, Kenya, on September 24, shortly before an explosion was heard. Sounds of heavy gunfire erupted from the mall Tuesday, even as authorities said they had the building under their control. But four days after Al-Shabaab terrorists stormed the swanky mall, several gunmen -- including snipers -- were still inside, two senior officials said. Hide Caption 3 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – Kenyan Defense Forces walk near the mall on Monday, September 23. Hide Caption 4 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – Kenyan Defense Forces leave the mall on September 23. Hide Caption 5 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – Stephen, center, is comforted by relatives as he waits for the post mortem exam of his father, who was killed in Saturday's attack at the mall. Hide Caption 6 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – A Kenyan police officer guards the entrance of a building near the mall on September 23. Hide Caption 7 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – A Kenyan security officer takes cover as gunfire and explosions are heard from the mall on September 23. Hide Caption 8 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – Heavy smoke rises from the Westgate Shopping Mall on September 23. Hide Caption 9 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – Medics take cover behind a tree as gunfire and explosions are heard from the Westgate Mall on September 23. Hide Caption 10 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – A Kenyan police security officer runs for cover as heavy smoke rises from the mall on September 23. Hide Caption 11 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – A paramedic runs for cover outside the mall on September 23. Hide Caption 12 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – People run for cover outside the mall after heavy shooting started on September 23. Hide Caption 13 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – Kenyan security forces crouch behind a wall outside the mall on September 23. Hide Caption 14 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – Soldiers take cover after gunfire near the mall on September 23. Hide Caption 15 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – Kenyan paramilitary police officers patrol the area near the mall on Sunday, September 22. Hide Caption 16 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – Soldiers from the Kenya Defense Forces arrive outside the Westgate Mall on September 22. Hide Caption 17 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – A woman shields a baby as a soldier stands guard inside the Westgate Mall on Saturday, September 21. Hide Caption 18 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – A rescue worker helps a child outside the mall. Hide Caption 19 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – People who had been hiding inside the mall during the gunfire flee the scene. Hide Caption 20 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – An armed official takes a shooting position inside the mall. Hide Caption 21 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – An armed official crouches on September 21. Hide Caption 22 of 51 Photos: Photos: Kenya mall attack Hide Caption 23 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – Bodies lie on the ground inside the mall. Hide Caption 24 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – Men help a wounded woman outside the mall. Hide Caption 25 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – Officials carry an injured man in the mall. Hide Caption 26 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – Soldiers move up stairs inside the Westgate Mall. Hide Caption 27 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – Armed police leave after entering the mall. At least one suspect has been killed, a government official said. Police have said another suspected gunman has been detained at a Nairobi hospital. Hide Caption 28 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – Armed police take cover behind escalators as smoke fills the air. Witnesses say tear gas was thrown in the corridors. Hide Caption 29 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – A woman who had been hiding during the attack runs for cover after armed police enter the mall. Hide Caption 30 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – A body is seen on the floor inside the smoke-filled four-story mall. Hide Caption 31 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – An injured person is helped on arrival at the Aga Khan Hospital in Nairobi after the attack at the upscale mall. Hide Caption 32 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – A soldier directs people up a stairway inside the Westgate on September 21. Hide Caption 33 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – An injured man is wheeled into the Aga Khan Hospital in Nairobi. Hide Caption 34 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – People run from the Westgate Mall. Hide Caption 35 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – A Kenyan woman is helped to safety after the masked gunmen stormed the upscale mall and sprayed gunfire on shoppers and staff. Hide Caption 36 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – Crowds gather outside the upscale shopping mall. The interior ministry urges Kenyans to keep off the roads near the mall so police can ensure everyone inside has been evacuated to safety. Hide Caption 37 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – A policeman carries a baby to safety. Authorities said multiple shooters were at the scene. Hide Caption 38 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – Bodies lie outside the shopping mall. Hide Caption 39 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – A security officer helps a wounded woman outside. Hide Caption 40 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – Elaine Dang of San Diego is helped to safety after the attack. The military asked local media not to televise anything live because the gunmen are watching the screens in the mall. Hide Caption 41 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – Paramedics treat an injured man outside the mall. Hide Caption 42 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – Medical personnel carry a body away. Hide Caption 43 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – A body lies outside the mall. Gunmen shot people outside the mall as they entered it Hide Caption 44 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – A woman is pulled by a shopping cart to an ambulance. Hide Caption 45 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – A wounded man is escorted outside the mall. Hide Caption 46 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – A police officer carries a baby as people keep low and run to safety. Crowds dashed down the streets as soldiers in military fatigues, guns cocked, crawled under cars to get closer to the mall. Hide Caption 47 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – People run away from the scene. Hide Caption 48 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – Armed Kenyan forces take position to secure the area around the shopping mall as ambulances move in to carry the injured. Hide Caption 49 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – A woman reacts after she is rescued from the mall. Hide Caption 50 of 51 Photos: Photos: Kenya mall attack Kenya mall attack – A couple flee the area. As night fell, authorities said they had cornered the gunmen in the mall. Hide Caption 51 of 51

Photos: Photos: Kenya mourns mall shooting victims Photos: Photos: Kenya mourns mall shooting victims Kenya mourns mall shooting victims – A woman shouts during a special prayer at the Legio Maria African Mission church in Nairobi, Kenya, on Sunday, September 29, for the victims of the Westgate Shopping Mall shooting. Hide Caption 1 of 21 Photos: Photos: Kenya mourns mall shooting victims Kenya mourns mall shooting victims – Kenyan Jainists dance as they reach the end of a continuous 24-hour prayer session on September 29 at the Oshwal Center next to the Westgate Mall. Hide Caption 2 of 21 Photos: Photos: Kenya mourns mall shooting victims Kenya mourns mall shooting victims – Muslim men pray at a memorial service outside the Westgate Mall on September 29. Hide Caption 3 of 21 Photos: Photos: Kenya mourns mall shooting victims Kenya mourns mall shooting victims – People lay flowers and light candles to pay their respects in front of the Westgate mall on Saturday, September 28, one week after the mass shooting. Hide Caption 4 of 21 Photos: Photos: Kenya mourns mall shooting victims Kenya mourns mall shooting victims – A man sits at a table with photos of Mbugua Mwangi and Mwangi's fiancee, Rosemary Wahito, during their funeral service in Nairobi on Friday, September 27. Mwangi was the nephew of Kenyan President Uhuru Kenyatta's. Hide Caption 5 of 21 Photos: Photos: Kenya mourns mall shooting victims Kenya mourns mall shooting victims – Mourners attend a prayer service held for Mwangi and Wahito at St. Andrews Church in Nairobi on Thursday, September 26. Hide Caption 6 of 21 Photos: Photos: Kenya mourns mall shooting victims Kenya mourns mall shooting victims – Family members light a funeral pyre at the Sikh funeral of Mitul Shah, the president of a football team in Kenya, in Nairobi on September 26. Hide Caption 7 of 21 Photos: Photos: Kenya mourns mall shooting victims Kenya mourns mall shooting victims – An Indian man prays on September 26 by the grave of Kenyan journalist Ruhila Adatia Sood, who was killed by gunmen at the Westgate mall, during her funeral in Nairobi. Hide Caption 8 of 21 Photos: Photos: Kenya mourns mall shooting victims Kenya mourns mall shooting victims – Relatives and friends carry the coffin of Ruhila Adatia Sood, a Radio Africa television and radio presenter, during her funeral in Nairobi on September 26. Hide Caption 9 of 21 Photos: Photos: Kenya mourns mall shooting victims Kenya mourns mall shooting victims – Manish Mashru looks at the ashes of his daughter Neha Mashru on September 26. Hide Caption 10 of 21 Photos: Photos: Kenya mourns mall shooting victims Kenya mourns mall shooting victims – Family members pay their last respects at the funeral of Mitul Amritlal Shah at the Hindu Crematorium in Nairobi on September 26. Hide Caption 11 of 21 Photos: Photos: Kenya mourns mall shooting victims Kenya mourns mall shooting victims – Mourners on Thursday, September 26, observe the body of Sridhar Natarajan, who was killed during the Westgate Shopping Mall attack in Nairobi, Kenya. Hide Caption 12 of 21 Photos: Photos: Kenya mourns mall shooting victims Kenya mourns mall shooting victims – Sanjivi Natarajan, brother of Sridhar Natarajan, mourns during his brother's cremation in Nairobi on September 26. Hide Caption 13 of 21 Photos: Photos: Kenya mourns mall shooting victims Kenya mourns mall shooting victims – Relatives carry a coffin Wednesday, September 25, during a funeral procession for Selima Merali and her daughter Nuriana Merali, who were killed by gunmen in the attack at the Westgate Shopping Mall in Nairobi, Kenya. Hide Caption 14 of 21 Photos: Photos: Kenya mourns mall shooting victims Kenya mourns mall shooting victims – Mary Italo, center, grieves for her son Thomas Abayo Italo as they wait to receive his body at the mortuary in Nairobi on September 25. Hide Caption 15 of 21 Photos: Photos: Kenya mourns mall shooting victims Kenya mourns mall shooting victims – Members of the Kenyan Sikh community cremate a woman and boy on September 25. Hide Caption 16 of 21 Photos: Photos: Kenya mourns mall shooting victims Kenya mourns mall shooting victims – A cemetery worker gathers his tools after a funeral on September 25. Hide Caption 17 of 21 Photos: Photos: Kenya mourns mall shooting victims Kenya mourns mall shooting victims – A Kenyan woman brings flowers to a funeral on September 25. Hide Caption 18 of 21 Photos: Photos: Kenya mourns mall shooting victims Kenya mourns mall shooting victims – A relative mourns during the funeral service for Selima Merali and Nuriana Merali on September 25. Hide Caption 19 of 21 Photos: Photos: Kenya mourns mall shooting victims Kenya mourns mall shooting victims – People gather for a funeral on September 25. Hide Caption 20 of 21 Photos: Photos: Kenya mourns mall shooting victims Kenya mourns mall shooting victims – A street vendor makes floral wreaths outside a mortuary in Nairobi on September 25. Hide Caption 21 of 21

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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.