A video has emerged showing four gunmen involved in the massacre at the Westgate shopping centre in Nairobi, Kenya.

It shows the attackers walking through a storeroom in the mall, searching other adjacent rooms and walking near the cash registers of a supermarket. The CCTV pictures were appaarently shot as the massacre was ongoing, while many victims remained remained trapped inside the mall.

Kenya's military spokesman, Major Emmanuel Chirchir, named four people implicated in the attack. He gave their names as Abu Baara al-Sudani, Omar Nabhan, Khattab al-Kene and Umayr.

Al-Sudani, from Sudan, was the leader of the group inside the mall and had been trained by al Qaeda, said Chirchir. He is "an experienced fighter and sharpshooter", he added.

Nabhan, a Kenyan of Arab origin, was born in Mombasa and travelled to Somalia with his uncle at the age of 16, said the spokesman.

The third attacker, Al-Kene, is said to be Somali from the capital Mogadishu, and is linked to al Shabaab Islamist militants.

The other names of the fourth attacker, Umayr, as well as his nationality and history were "not yet identified", he said.

Sky News reported that Al-Kene and Umayr are known members of al Hijra, a Kenyan extremist group affiliated with al Shabaab, according to Matt Bryden, former head of the United Nations Monitoring Group on Somalia.

Kenya's government initially said 10 to 15 attackers were involved in the assault, which left at least 67 people dead, and a further 39 people still unaccounted for, according to the Red Cross.

However, police now believe between four and six people took part in the murderous siege.

"From what we have now that is coming out of the investigation, the number of attackers was between four to six," police chief David Kimaiyo told Kenyan television station KTN. "None of them managed to escape from the building after the attack," he said.

Kimaiyo also confirmed that wanted British "White Widow" Samantha Lewthwaite – who had been reported to have been one of the attackers – was not involved.

He said: "On Samantha we have also established that she was not part of the attackers in the building. There was no woman."

Somali islamist group al Shabaab has said it carried out the attack in retaliation for Kenya sending troops into Somalia nearly two years ago.

The group has promised more attacks inside Kenya unless those troops are withdrawn.

Kenyan President Uhuru Kenyatta has vowed to continue the military mission inside Somalia despite the terror attack.