Witness photo of security officer outside the mall

NAIROBI, Kenya (CNN) -- Some answers may be revealed in blood-stained halls or deep in the rubble of Nairobi's Westgate Mall. Others may never be known.

That's the reality for investigators and the people of Kenya on Wednesday, still coming to grips with a vicious attack and armed standoff that ended a day earlier.

At least 61 civilians and six Kenyan security officers died in the attack and rescue efforts, President Uhuru Kenyatta said Tuesday, but the death toll will rise as recovery workers retrieve bodies buried in the rubble of the partially collapsed mall.

Kenyan forces killed five of the terrorists, and 11 other people are in custody for possible links to the attacks, Kenyatta said, declaring that his country had "ashamed and defeated" the attackers.

But even though Kenyatta declared the siege over, an immense amount of work remains to learn how Al-Shabaab, a terror group thought to be badly bruised by recent losses of territory in its Somalian homeland, was able to pull off such a well-coordinated and brazen attack.

How did they do it?

It started on Saturday when the attackers stormed into the upscale mall and began shooting. A senior Kenyan government official said they took "very few" people captive; the attackers were primarily out for blood.

"They were not interested in hostage-taking," the official said. "They only wanted to kill."

The attackers were well-enough equipped to kill dozens of civilians, then fend off Kenyan security forces for four days -- not the sort of action that can be pulled off on a whim.

That raises a number of questions: How could such a significant plot, involving travel arrangements, arms transfers and other details, have escaped the attention of intelligence officials? Did the attackers have inside help, either at the mall or within security forces?

So far, Kenyan and U.S. authorities aren't answering such questions, certainly not publicly. But The New York Times, citing unnamed American security officials, said Wednesday that it appeared the attack had been well-planned and that the attackers must have had access to storage at the mall to stash their weapons.

One official quoted by the newspaper said militants had access to a heavy belt-fed machine gun that couldn't have been openly carried into the mall without attracting notice.

Who were the attackers?

Kenyan authorities have said 10 to 15 attackers were believed to be involved.

One of the attackers was Dutch, another British, Kenya State House spokesman Manoah Esipisu told CNN on Wednesday.

Al-Shabaab had previously claimed that Americans were involved in the attack, a claim Kenyatta noted Tuesday but said has yet to be verified. Esipisu said Wednesday that Kenyan authorities believe attackers of "a few other nationalities" were involved.

Kenyan Interior Minister Joseph Ole Lenku told reporters Wednesday that authorities cannot confirm the nationalities of the terrorists until forensic testing is complete. He said the United States, Israel, Britain, Germany, and Canada are helping in the mall forensic investigation.

Citing unnamed American officials, The Times reported that American officials believed Al-Shabaab may have recruited the attackers from the United States and other Western countries for their skill in English, which is widely spoken in Kenya.

One of the more tantalizing rumors suggests the involvement of Samantha Lewthwaite, a British woman whose husband killed himself in a 2005 London suicide bombing. Known as the "White Widow," Lewthwaite has been wanted by international counter-terrorism officials since authorities found bomb-making materials in her Mombasa, Kenya, apartment in 2011. She vanished shortly before a raid.

A senior Kenyan official said Tuesday that a woman was involved in the attack. Esipisu said Wednesday that authorities can't say much about who the woman was or what she was doing.

"What we've been told by multiple witnesses is that they saw a woman. We have also been told that if it is the same woman that they say they saw, that she would have been killed very early on in the attack," Esipisu said." We don't know for sure that we had a woman. And secondly, because of the bodies trapped under the rubble, we don't know if she is who everyone says she might be."

Where are they?

Some of them are dead, inside the rubble of the partially collapsed mall, Kenyatta said.

But while he said five of the terrorists had been killed by Kenyan forces and 11 people were in custody, it was not clear if all of the attackers had been accounted for, or if some may have been able to slip out in the chaos.

While a senior Kenyan official said forces were able to drive two attackers trying to escape by car back inside the mall, it's unclear if any others might have been able to successfully elude authorities early in the crisis. Others could have escaped by posing as civilians, perhaps after ditching their weapons and changing clothes.

On Wednesday, a high-level source who asked for anonymity told CNN that Kenyan counterterrorism police had arrested a British national of Somalia descent who had injuries on his face and was acting suspiciously as he tried to board a Turkish Airlines flight. It's not clear if Kenyan authorities suspect the man of being inside the mall during the attack, but authorities found they had no record of the man's entry into the country, the source said.

Kenyatta, whose country boasts deep counterterrorism ties to the United States, vowed to track down and punish the attack's perpetrators.

"These cowards will meet justice, as will their accomplices and patrons wherever they are," he said Tuesday.