Police said they had arrested nine suspects, in addition to the two arrested on Wednesday, including a man initially thought to have been among the dead attackers.
Kenyan police aided by bomb experts and sniffer dogs on Thursday resumed their search of the Nairobi hotel complex as police arrested nine more suspects over the attack which left 21 dead and 28 injured.
A police source, speaking on condition of anonymity, said they were confident there were no more people trapped inside the hotel or surrounding office buildings after the 20-hour assault unleashed on Tuesday, during which some 700 civilians were rescued.
"We are confident that we have no more people there," the officer told AFP, "but in a situation like this, you are not done until you are done."
"We are back in with sniffer dogs, and bomb experts are checking through because yesterday we found grenades left by those people."
Police warned the public there may be loud controlled explosions as they continued their sweep of the hotel.
Five gunmen with the al Qaeda-linked Somali militant group al Shabaab attacked the DusitD2 hotel and office complex on Tuesday afternoon.
Chilling CCTV footage showed one of the attackers lingering in front of the terrace of the Secret Garden restaurant before blowing himself up.
Four other attackers were shot dead by police during the operation to secure the hotel.
TRT World's Abdi Osman reports.
'A breakthrough' in probe
Police said Thursday they had arrested nine suspects, in addition to the two arrested on Wednesday, including a man initially thought to have been among the dead attackers.
Ali Salim Gichunge was rounded up after a raid at his house in the Ruaka suburb on Wednesday where he lived with his wife Violet Kemunto Omwoyo who is also being held by police.
"He is linked to the attack but he is not among the dead," a police source said. "His car was used and he had hosted the attackers. His house in Ruaka was used in the planning, that is why he is very important to us in this investigation. "
"It is a breakthrough for us. With his car on site, everyone believed he must be dead, but the description from neighbours and his wife has led to his arrest."
The other suspects include men picked up in the coastal city of Mombasa and western Kenya who were all communicating with "associates" in Somalia.
The police source earlier said detectives had discovered a "huge hole dug in one of the rooms where guns were stored."
"Neighbours have told us the couple was planning to move out because they had even put up their items for sale through the community's social network."
The Standard newspaper said Omwoyo had written: "We are moving out of Nairobi this week" on her ad selling clothes and furniture.
Al Shabaab said it was acting to avenge the decision by US President Donald Trump to declare Jerusalem as Israel's capital, according to the SITE monitoring group.
A statement from the Vatican said Pope Francis was "deeply saddened to learn of the loss of life and injury following the attack in a hotel complex in Nairobi."
"He extends heartfelt condolences to all Kenyans, in particular the families of the deceased and all those injured."
Missing accounted for
Among the victims who died in the attack were two ethnic Somali Kenyans who worked on a project called the Somalia Stability Fund, a Kenyan football blogger and a policeman responding to the scene.
An American working for a consulting and investment firm, who survived the 9/11 attacks in the United States, and a dual British-South African development worker were also among those killed.
As the attack unfolded, scores of terrified civilians barricaded themselves inside toilets and offices for hours.
Some were killed at the Secret Garden restaurant, while another six bodies were found on the third floor of the hotel, where one of the gun battles took place.
The Red Cross said that 19 people who had been reported missing to them had since been traced.
Several private citizens and foreign security forces took part in the rescue operation.
British media reports said a former special forces soldier who was out shopping when the attack happened jumped in to help Kenyan special forces.
Kenya's Daily Nation newspaper hailed the "sweet victory over terrorism" as praise poured in for the swift and professional response from security services.
This stood in stark contrast to the scorn heaped on disorganised police and army officers who staged a chaotic intervention when Shabaab attacked the Westgate mall in 2013, leaving 67 dead.
"There was a sense of triumph despite adversity," read the newspaper's editorial.
"Security teams were well-coordinated under the General Service Unit; there was a centralised chain of command... clearly lessons have been learned."
Al Shabaab has repeatedly targeted Kenya over the presence of its troops in Somalia.