Scores more bodies are believed to be buried under rubble in Kenya's Westgate shopping mall as gruesome details continued to emerge on Wednesday from the aftermath of the four-day terrorist siege.

As one morgue in Nairobi said it was preparing for up to 160 new corpses, an intelligence source told the Guardian there was evidence that the number of bodies buried under rubble could be in the hundreds, adding that at least one attacker was still engaged in conflict with security forces inside the mall, five days after the shopping centre was stormed on a busy Saturday morning.

"One attacker is still alive," said the source, who is involved in the rescue operation inside Westgate, and who asked not to be named, adding: "There were 200 workers in Nakumatt [supermarket] and 800 people shopping. The walls of Nakumatt collapsed. There are a lot of bodies inside."

Al-Shabaab, the Somali-based, al-Qaida-linked group behind the attack, claimed the government had carried out a "demolition" of the building, burying 137 hostages.

The Kenyan government rejected claims that any militants remained alive, or that they had demolished part of the building, but said the mall's upper–level car park had collapsed, bringing the second level down on to the ground floor on top of at least eight civilians and one or more attackers.

Government spokesman Manoah Esipisu said the number of dead – which stood at 67 – already included the bodies of those buried beneath the rubble, and denied the figure would rise further.

Kenya's interior minister, Joseph Ole Lenku, said the process of determining the number of dead and gathering evidence from the mall could take a further week. He said the US, Israel, Britain, Germany, Canada and Interpol were helping with the investigation.

"This morning, forensic experts began sifting through the rubble at the Westgate mall," Lenku said at a press conference close to the centre.

"This process involves finger-printing, DNA and ballistics examination. We do not expect the numbers of the dead to increase in any significant way. Yes, there could still be bodies trapped in the rubble. We can only confirm that after we have gone through the rubble."

The US ambassador to Kenya, Robert Godec, said US experts were providing technical support and equipment to Kenyan security forces and paramedics.

Sikhs come together in Nairobi, Kenya, to cremate a grandmother and son who were killed in the Westgate mall attack. Photograph: Kabir Dhanji/EPA

Six British nationals are among the dead. The British government said that a number had also been injured and were still in Kenya receiving treatment, but declined to provide any further information. A spokesman also said he could not rule out the possibility that one or two British nationals, or dual nationals, were still missing.

As questions remained about whether any of the attackers was from the UK, the British high commissioner in Kenya, Christian Turner, confirmed that a Briton had been arrested but said: "We do not believe that the individual was of central interest to this operation."

The statement came after intense speculation as to the role of foreign al-Shabaab recruits in carrying out the attack, following reports that a British woman and two or three Americans may have been involved. Neither country has ruled out the claims, but the US attorney general, Eric Holder, said there had been no verification that Americans were involved.

"Most of the jihadis are diaspora Somalis who have been living in the west," said a Kenyan intelligence source, who did not want to be named. "We know this was an operation by a faction of al-Shabaab, with Somalis, probably working with al-Qaida."

More details were beginning to emerge about the attack itself. One military expert told the Guardian he had information that al-Shabaab had set up inside the mall, where they had stockpiled weapons and ammunition in preparation for the siege.

"This is a terrorist cell that has been operating in Kenya for a while," said Colonel Benjamin Muema, security expert in Nairobi. "It is a cell that knows the mall, had a blueprint of the building, and was a step ahead of the security forces.

"I believe that this group has had premises within the mall, and that explains how they were able to continue for four days without running out of bombs, grenades and explosive devices."

The government did not rule out that a female attacker was involved. "Regarding the identity of the terrorists, we want to request the public and the international community to allow our experts to undertake the forensics," said Lenku.

Al-Shabaab also made claims about the way the attack was carried out, stating in an email exchange with Associated Press that foreigners were a "legitimate target" and that Muslims had been spared.

Late on Wednesday night, al Shabaab's leader for the first time confirmed claims by his group's members that it was behind the attack on the mall, Reuters reported.

In an audio posted on the al-Shabaab-linked website, Ahmed Godane, also known as Mukhtar Abu al-Zubayr, said the attack was in retaliation for Kenya's incursion in October 2011 into southern Somalia to crush the insurgents.

"Take your troops out or prepare for a long-lasting war, blood, destruction and evacuation," Godane said in the message delivered in the Somali language and apparently directed at the Kenyan government. Kenyan troops are fighting alongside African peacekeepers against the militants in Somalia.

Al Shabaab had threatened revenge since Kenyan troops joined the conflict.

"You are part of the massacre Kenya carried out in Kismayu and in other towns because you had elected your politicians. The tax you pay is used to arm Uhuru [Kenyatta] forces that massacre Muslims. You had supported the fight against us," Godane said.

As conflicting claims continued surrounding the details of the attack, coroners dressed head-to-toe in protective clothing were busy clearing inside the city's main morgue – a single storey building surrounded by neat hedges and leafy gardens that did little to mask the strong odour of decaying corpses.

"We are preparing to receive up to 160 bodies here", said Marion Githinnaji, member of the Nairobi city assembly in charge of the Nairobi city council mortuary. "This morgue acts as a clearing house. People come here to identify the bodies, and we do the fingerprints and processes, then the bodies and police files are moved to private facilities because there is not enough space."

"We have coroners there at the mall but because the military are handling it, we haven't been able to remove any of the bodies yet. They haven't told us exactly how many to expect."

An elderly lady, clutching a bottle of water and wearing a lime green jacket and matching headband, who did not want to be named, had identified a body as her nephew. She spoke through tears as she criticised the authorities for their handling of her nephew's case.

"We came here yesterday to identify the body, and we came again at 8am today for the postmortem," she said. "The police say that the doctor refuses to do the postmortem, and the doctor says it's the police. What is this?"

There is mounting criticism of the Kenyan authorities' handling of the victims and of the attack itself.

Muema said it represented a serious failure of intelligence. "There was no intelligence, nobody knew that this attack was going to happen, and everyone was caught by surprise.

"Then the initial response was to deal with it as a robbery, not as a terrorist attack, which only served to aggravate the situation.

"And then when the magnitude of the attack was revealed, the agencies were not unified under a single command structure. No one knew who anyone else was and there may well have been friendly fire."