NAIROBI, Kenya — Kenyan and American investigators began a sweeping inquiry on Wednesday into a shopping mall massacre here that killed scores of people, sifting through rubble, studying closed-circuit television footage and bringing in more resources to identify the attackers.
Tyler Hicks/The New York Times
A grief counselor, left, comforted Agnes Mutua, who had just identified her nephew, Christopher Chewa, 26, on Wednesday at a mortuary in Nairobi.


“The next phase really is making sure we know what’s under the rubble,” said a government spokesman, Manoah Esipisu. “Forensic people need to be able to clear that rubble and examine the evidence beneath it.”
The investigative work began a day after President Uhuru Kenyatta of Kenya declared that the four-day siege of the Westgate mall had ended, saying that the government had finally “ashamed and defeated our attackers” and that the last militants still holed up inside had been killed.
Joseph Ole Lenku, the Kenyan interior secretary, said at a news briefing on Wednesday that the death toll, now at 72, was not expected to rise significantly, Reuters reported, although the Kenyan Red Cross has said scores of people are still missing in the debris.
The massacre plot was hatched weeks or months ago on Somali soil, by an “external operations arm” of the Shabab, a militant Islamist group based there, according to American security officials. A team of English-speaking foreign fighters was carefully selected, along with the target: Westgate, a gleaming upscale mall popular with expatriates and Nairobi’s rising middle class.
The picture that is emerging from the American officials suggests careful planning: The building’s blueprints were studied, down to the ventilation ducts. The attack was rehearsed and the team dispatched, slipping undetected through Kenya’s porous borders, often patrolled by underpaid — and deeply corrupt — border guards.
A day or two before the attack, powerful belt-fed machine guns were secretly stashed in a shop in the mall with the help of a colluding employee, officials say. At least one militant had even packed a change of clothes so he could slip out with fleeing civilians after the killings.
“They had people in there, they had stuff inside there,” said an American security official who asked not to be identified because he was not authorized to speak publicly. “This was all ready to go when the shooters walked in.”
Mr. Kenyatta said that intelligence reports had suggested that a British woman and two or three American citizens were involved, but that he could not confirm those reports. American officials said they had not determined the identities of the attackers and were awaiting DNA tests and footage from the mall’s security cameras. But they said the massacre had been meticulously planned to draw maximum exposure.
On Wednesday, Kenya began an official three-day period of mourning to mark one of the most unsettling episodes in its recent history. The authorities in Kenya, widely perceived as an oasis of peace and prosperity in a troubled region, are struggling to answer how 10 to 15 Islamist extremists could lay siege to a shopping mall, then hold off security forces for days.
On multiple occasions, the Kenyan government said the mall was under its control, only to have fighting burst out again. Earlier on Tuesday, the Shabab, which has taken responsibility for the attack, bragged in a Twitter message that its fighters were “still holding their ground.”
Western security officials fear that several fighters slipped out of the mall during the mayhem of the attack, dropping their guns and disguising themselves as civilians, an account echoed by some witnesses.
The way the attack was carried out may be related to a rift between Omar Hammami, a Shabab fighter who grew up in Alabama and became a phantomlike figure across the Somali deserts, and the group’s emir, Ahmed Abdi Godane. Mr. Hammami — known by his nom de guerre, Abu Mansoor al-Amriki, “the American” — was reportedly fatally shotby another wing of the Shabab less than two weeks ago, although he has been reported dead before only to resurface alive.
One reason for the rift was Mr. Hammami’s complaints that the Shabab had become too brutal toward fellow Muslims under Mr. Godane’s leadership. That brutality, Mr. Hammami said, was the reason the Shabab had become so unpopular in Somalia and had lost so much territory recently.
Witnesses to the siege in the mall have said the militants urged Muslims to get out before the shooting started, and Stig Jarle Hansen, a Norwegian researcher who has published a book on the Shabab, said the rift might explain why the militants decided to spare Muslims. In the past, the Shabab have killed countless Muslims in Somalia with suicide bombs and buried Muslim girls up to their necks in sand and stoned them.
“Even Osama bin Laden criticized Godane for being too harsh,” Mr. Hansen said. “This attack might have been Godane’s way of saying, ‘See, I’m not so harsh — to Muslims.' ”
Much speculation has swirled around the question of whether any of the heavily armed militants were women.
Kenyan officials initially asserted that there had been no women among the attackers, but on Tuesday Mr. Kenyatta seemed to revive the possibility that one was a British woman.
Several intelligence analysts in Nairobi speculated that she was Samantha Lewthwaite, a Muslim convert who had been married to one of the four suicide bombers who struck the London transit system on July 7, 2005, killing 52 people and themselves.
Kenyan authorities suspect that Ms. Lewthwaite had risen up through the ranks of extremist groups and was leading a terrorism cell on the Kenyan coast; though they almost swooped in on her in 2011, she escaped. In Kenya and in Britain, she is now known as “the white widow.”
British authorities confirmed on Wednesday that a British citizen had been detained in Nairobi. The Daily Mail newspaper said the 35-year-old man, believed to be of Somali origin, was arrested on Monday trying to board a Turkish Airlines flight to Turkey. His face was bruised, he was wearing dark glasses and he acted suspiciously, the newspaper said.
Asked to comment on the report, a Foreign Office spokeswoman, speaking in return for anonymity under departmental rules, confirmed the arrest and said the British authorities were offering “standard” consular assistance.
Nicholas Kulish and Jeffrey Gettleman reported from Nairobi, and Alan Cowell from London. Steven Erlanger contributed reporting from London.