Forensic investigators began a gruesome search of a wrecked mall in Kenya's capital on Wednesday as questions remained over who exactly was behind a deadly siege there and how it was ended by security forces.
The United States, Israel, Britain and Canada are among the countries helping Kenyan authorities identify those responsible for the four-day attack in which at least 72 people were killed, Kenya’s interior minister Joseph Ole Lenku told reporters.
The FBI was on the scene soon after the attack and another contingent, including the Evidence Response Team and bomb technicians, is ready to fly to Kenya, officials said.
“We have moved to the next phase,” Ole Lenku said, adding that security had been stepped up at the country’s borders.
Ole Lenku was still unable to confirm reports – including comments by one of his own government colleagues - that American and British citizens were among the attackers.
The leader of the al Qaeda-linked Somali Islamist militant group al Shabaab confirmed claims by members of his group that it was behind the attack Saturday on Nairobi’s Westgate shopping complex. Ahmed Godane, also known as Mukhtar Abu al-Zubayr, said it was in protest of Kenya's October 2011 incursion into southern Somalia to attack the insurgents, Reuters reported.
On Wednesday, al-Shabaab told The Associated Press in an email exchange that foreigners were a "legitimate target." Gunmen "carried out a meticulous vetting process," separating Muslims out so they would not be harmed and letting them go, the extremist group confirmed to AP on Wednesday.
Witnesses said the gunmen rounded up people, asked questions about Islam that a Muslim would know, then let Muslims go.
There were new questions Wednesday about how the siege was brought under control, and what caused the mall to partially collapse before rescuers could enter.
Al-Shabaab used an unverified Twitter account to claim that Kenyan forces had used “chemical agents” to end the siege in a “morally reprehensible manner.” Al-Shabaab regularly changes its Twitter handle as its accounts are frequently suspended.
It also claimed the true death toll in the attack was 137, although Ole Lenku said he was not expecting the official death toll of 67 to rise significantly.
At least five attackers were killed, and Ole Lenku said he believed the remaining bodies inside the mall were those of other attackers. Ten attackers were captured alive and arrested.
A report in the Kenya Standard newspaper said authorities were investigating whether the attackers had rented a shop in the mall in order to gain knowledge of the building.
A senior U.S. counter-terrorism official told NBC News that it's "more than plausible" that the perpetrators undertook extensive preparations, even rehearsals, before launching the assault.
"Al-Shabaab makes exacting preparations," the official said, noting that the group spent six months gearing up for a 2010 attack on World Cup fans in Uganda that killed 74 people.
Safe houses were rented and explosive materials were transported through Kenya and Somalia for that attack, which seemed to target Christians and expatriates, the official said.
Amina Mohamed, Kenya’s foreign minister, told PBS Newshour late Monday that Americans were among the attackers in the mall massacre — "young men, about between maybe 18 and 19" years old who lived in "in Minnesota and one other place."
However, Attorney General Eric Holder said Wednesday there was still no verification that Americans were involved, but that U.S. officials were working with their Kenyan counterparts to investigate the attack.
St. Paul, Minn., Police Chief Tom Smith said his department had "talked with the FBI throughout the weekend” about the Kenya attack but no links had been confirmed.
A British woman was also involved, Mohamed alleged – fueling speculation that Samantha Lewthwaite, the widow of a suicide bomber who attacked London's transit system in 2005, was linked to the Kenya siege.
A British man was arrested at Nairobi’s airport late Tuesday, the U.K. Foreign Office said – but Britain’s High Commissioner in Kenya, Christian Turner, later said the 35-year-old was “not of significant interest" to the mall attack investigation, Reuters reported.
At Nairobi’s main morgue, relatives and friends searched Wednesday for people missing since the massacre began.
Vehicles including hearses and minibuses arrived with bodies, the Kenya Standard newspaper reported. “The smell of decomposition wafted out of the open windows of the unrefrigerated morgue, hitting visitors as they entered the car park,” the report said.
As the gruesome clean-up began, a fuller picture emerged of the horror inside the crowded mall when the attack began. Video footage showed a family playing dead to avoid harm, shoppers escaping, and a child being carried to safety.
Dorkas Moingee, a woman who was inside the mall at the time of the attack, described to TODAY the initial shock of the siege and the harrowing panic that followed.
“When I first head the gunshots I wasn’t sure that they were gunshots,” Moingee said. “I actually thought it was an earthquake.”
NBC News’ Tom Winter, Mary Murray and Emma Ong and The Associated Press contributed to this report.