A new report by the New York police into the attack on Nairobi's Westgate shopping mall has concluded that the raid was carried out by only four al-Shabab members, who may have escaped.
The report, released on Tuesday, disputed findings by the Kenyan authorities, who said that up to 15 attackers, including foreign nationals, were involved in the raid that killed at least 67 people last September.
The victims aged eight to 78 were shot dead during a terrifying assault claimed by Somalia's al-Qaeda-linked al-Shabab group.
The report said the last confirmed sighting of the attackers on the mall's CCTV system was on September 22 at 00:54 hours, twelve hours after the start of the attack.
"It is unknown if the terrorists were killed or escaped the mall. A major contributing factor to this uncertainty was the failure to maintain a secure perimeter around the mall," the report said.
Media speculated the attackers may have escaped in the chaos of the fighting, although security sources in Kenya said they died in a final stand-off with commandos.
It took Kenya almost four days to declare the mall safe. The day before doing so, Kenyan forces started a fire and collapsed a large part of the mall, the report added.
The conclusions are based on information obtained by New York police officers and unclassified open source information.
The New York police and Interpol are assisting Kenya in the investigation into the attack.
Interpol is also trying to identify four bodies suspected of being those of the gunmen. No identification has yet been announced.
The report also highlighted poor coordination between the Kenyan police and the army, which saw troops kill one police officer and wound the commander of a police team.
Most of the victims were probably killed in the first hour of the attack when the gunmen engaged in "wholesale killing" of those trying to hide in places like storerooms, the report said.
According to witnesses and CCTV footage, the gunmen did not want to take hostages, only kill a broad spectrum of people.
The New York police report also confirmed a "conscious but erratic attempt" at avoiding Muslim casualties but voiced fears over the possibility of future copycat attacks.
"It also clearly illustrates that armed assaults by terrorists on 'soft' targets such as a shopping mall are a simple, effective and easy to copy tactic," the report said.
Al-Shabab said the attack was a warning to Kenya to pull its troops out of southern Somalia, where they are fighting the rebel group as part of an African Union peacekeeping force.
Western officials suggest as many as 94 could have died in the attack, with some victims potentially remaining under tons of rubble after part of the mall's roof collapsed.
Four suspects are due in court next week over the attack, charged with supporting the armed group. They also face charges of entering Kenya illegally and obtaining false identification documents.