Kenya mall attack investigators try to identify victims, terrorists, and explain bloody conclusion

(CBS News) NAIROBI, Kenya - FBI agents helped Kenyan officials comb through evidence Thursday at the scene of the four-day siege on an upmarket shopping mall in Nairobi, looking for clues on the identities of both the victims and the terrorists.

CBS News correspondent Charlie D'Agata said the gaping hole left in the Westgate Mall, where three stories of the building collapsed near the end of the deadly siege, was still smouldering.

The whole area was being treated as a crime scene, and there were discrepancies in the death tolls being reported by the government, which was sticking to the 61 fatalities it confirmed on Monday, and the Kenya Red Cross, which raised its toll on Thursday to 71. More bodies were expected to come out of the crushed concrete and twisted metal.

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Just what caused building to come down, burying both hostages and militants beneath the rubble, has become a key question in the investigation.

D'Agata said he and other journalists, who were kept about 200 yards from the mall itself, heard large explosions on Monday. One government official said soldiers firing rocket propelled grenades knocked out a supporting column.

Video released by the Kenyan government shows the damage to the Westgate Mall
Video released by the Kenyan government shows the damage to the Westgate Mall. CBS News

That information wasn't confirmed, and a military explosives expert told CBS News it was highly unlikely than an RPG could have caused the damage done to the structure.

Forensic teams were at work collected fingerprints and DNA to identify attackers and victims. Ballistics experts were identifying the weapons used.

And in a crucial attempt to determine exactly how the al Qaeda-linked group al-Shabab, based across the border in Somalia, managed to carry out such a large-scale and well-armed attack, investigators were looking into reports that the gunmen might have rented a shop in the mall to stash their firearms and ammunition months earlier.

"Whether the terrorists hired a shop in the mall is a rumor," Interior Minister Joseph Ole Lenku told CBS News. "We will treat it as such, until the forensic exercise and any other investigation taking place prove it otherwise."

As more survivor accounts emerged, it seemed that whether people trapped in the mall lived or died was down largely to the whim of the gunmen.

Al-Shabab claimed Wednesday that the militants "carried out a meticulous vetting process" so Muslims would not be harmed. But there were Muslims among the dead and wounded.

Syed Ibrahim pleaded with one of the terrorists for his life, and was let go.

"I just told him to leave me alone, I've been injured," he told D'Agata. "So they let me go. They said, 'we won't do anything to you, go,' because I'd been shot already."