The four militants behind the Westgate mall attack in Kenya travelled into the country overland from Somalia in June, a Western official has told the BBC.

The senior official said they had entered at a "common entry point" and then stayed in Eastleigh, a suburb in Kenya's capital, Nairobi.

The attackers had been together in Somalia, where it is believed they had been trained, the official said.

Sixty-seven people died in the Nairobi shopping centre in September.

The Somali Islamist group al-Shabab said it carried out the attack in revenge for Kenya's presence on Somali territory as part of the African Union force in the country.

So far four foreigners have been charged in connection with the attack, accused by police of sheltering the attackers in their homes in Eastleigh - a Somali neighbourhood in Nairobi known as "Little Mogadishu".

Mohammed Ahmed Abdi, Liban Abdullah, Adan Adan and Hussein Hassan deny supporting a terrorist group.

'No Dadaab link'

The official, who is privy to investigation in Nairobi, said he was confident there were only four attackers as the authorities in Kenyan have been saying.

Two of those behind the four-day siege have been identified in court documents as Hassan Dhuhulow, believed to a Somali-Norwegian, and Mohammed Abdinur Said.

"They got to Kenya in June this year through a common entry point. They lived in Eastleigh," the official told the BBC.

"They were in Somalia prior to the attack being carried out. There were in Kenya for some time together."

The militants had what the official called a "local support network".

But he said there was no evidence yet that Samantha Lewthwaite, the British widow of one of the suicide bombers who attacked the London transport system in 2005, was involved either in the attack or in the support network.

The BBC's Africa security correspondent Moses Rono says this new information is a step closer to finding the true identities of the attackers and their links to al-Shabab.

But he says the question remains whether - or how - the four were supported by al-Hijra, the Kenyan-based group, also known as the Muslim Youth Centre, which is viewed as a close ally of al-Shabab.

It is also bound to re-focus attention on Eastleigh, which has suffered reprisal attacks from some who accuse the Somali community of harbouring terrorists, our reporter says.

Some politicians believe some in the sprawling neighbourhood host al-Shabab militants or their sympathisers.

The Dadaab refugee camp in north-eastern Kenya, which hosts more than 400,000 Somalis near the Somali border, has also been accused by Kenyan officials of having links to the militants.

"I would not link their entry to Dadaab. I would not suggest that they come in through Dadaab," the official said about how the attackers reached Kenya from Somalia.

He added that the four had travelled aboard in the past, but he could not divulge the countries except to exclude the UK and US.