(ABC News) -- U.S. federal agents have joined a team of international investigators to sift through the still smoking rubble of the Kenyan shopping mall that was the site of a four-day terrorist rampage, beginning the painstaking process of looking for clues about the attackers, officials said today.
Calling the investigation an "elaborate process," Joseph Ole Lenku, Kenya's interior secretary, said investigators from the U.S. and INTERPOL were already on the scene looking for fingerprint, DNA and ballistic evidence.
He said the investigation of the site will take at least a week, but investigators did not expect to find a significant number of victims still trapped in the rubble.
"We are convinced that there will be insignificant if any bodies still holed up there except for the terrorists," he told reporters today.
The investigation has now turned to how the terrorists, associated with the Somalia-base Islamist group al-Shabab, were able to execute the attack using machine guns, grenades and homemade bombs that killed at least 72 people.
An interior ministry spokesman said today that the band of assailants had rented a shop in the mall for three months, storing weapons and plotting Saturday's rampage.
Later that same day, Lenku said that claim was only a "rumor" until it had been verified by the investigation.
"As to whether they had a shop in the mall is something we cannot say categorically," Lenku said.
He said five suspects were killed and 10 taken into custody. Authorities, he said, were working to establish the identities and nationalities of the suspects.
A U.S. official briefed on the Nairobi investigation says American authorities still cannot confirm whether any Americans were involved in the Nairobi attack, a possibility that was raised by Kenya's President Uhuru Kenyatta and the country's foreign minister. Nairobi officials also said they were checking reports that a British woman took part in the assault.
The U.S. official says authorities intend to conduct the necessary analysis, including biometric analysis and fingerprints.
A former diplomat who lives in Nairobi and maintains Kenyan government and international security contacts told ABC News Tuesday night that intelligence services in Kenya had detected surveillance of Westgate Mall being conducted in the past year, but said hard intelligence it was being targeted for a terror attack wasn't uncovered.
"The intelligence hasn't been specific," the former diplomat told ABC News. The former diplomat described the al-Shabab attack as "strictly for revenge" because of Kenya's help in defeating the terror group al-Shabab in Somalia. The ex-diplomat said Muslims were "executed" alongside non-Muslims in Westgate. "It was completely indiscriminate murder," he said.
Kenya beings three days of national mourning today to remember the victims as troops dig through the rubble of the building, scouring for bodies of victims and terrorists buried after part of the complex collapsed.
Troops remain deployed at the Westgate mall following the conclusion of tense four-day siege, which began Saturday when armed members of al-Shabab opened fire on shoppers.
The death toll is expected to rise as Kenyan forces sift through the rubble after three floors of the mall collapsed Tuesday during the final hours of fighting.
Dorcas Mwangi said she hid from the attackers for four hours under a pile of suitcases. She says her brother texted her, warning the terrorists were killing non-Muslims. Her brother sent an Islamic prayer so she could memorize it.
"I was able to memorize it… In case they found me," she said.
American Bendita Malakia, now back on U.S. soil, says she hid in the backroom of a home goods store for nearly five hours while the terrorists controlled the mall in the opening hours of the siege.
"We just heard explosions and starting crawling out. It was very, very, very scary," Malakia recounted.
A haunting image emerged Tuesday showing a family playing dead until a plain clothed police officer came to their rescue.