"How did he get to where he is?" said a private security official in Kenya, who asked not to be identified for operational reasons.
Ng'ang'a, a taxi driver, reportedly took the attackers to several locations while coordinating their attacks.
"In the course of investigation of this nature, close family members of such people are crucial because they hold crucial information", the source said.
The attack was carried out by the Islamist-Somali extremist group al-Shabab.
Kenyan authorities say 21 people, including one police officer, were killed by the attackers, one of whom blew himself up beside a restaurant.
A judge has ordered the five suspects held for 30 days while authorities look into the assault on Nairobi's dusitD2 hotel complex.
Suspects who appeared in court were identified as Joel Nganga Wainaina, Oliver Kanyango Muthee, Gladys Kaari Justus, Guleid Abdihakim and Osman Ibrahim.
Director of public prosecutions Noordin Haji says the investigations are complex and transnational and require sufficient time to uncover the entire "criminal syndicate".
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The pair's father is an officer in the Kenyan military, she told Reuters.
The police official did not say when the other seven were arrested. He spoke on condition of anonymity because he was not authorized to speak to the media.
Stepping in front of the cameras in the wake of this week's deadly siege of a Nairobi hotel and office complex, Kenyan President Uhuru Kenyatta pledged to finally root out the Somalia-based militants who had claimed the attack.
The Somali leader whose country is facing terror attacks from terrorist group al-Shabab also condemned the attack which seemed to target expatriates working for multinational companies located in Kenya.
Chairman of the Kenya Tourism Federation Mohammed Hersi said that the situation in Nairobi was calm and security remained stable with smooth transport and normal schedules including the global airports in both Nairobi and the coastal city of Mombasa.
"Where are you guys?" the agitated bomber said at least a couple of times, according to Abdullahi Ogelo, the employee.
In the television interview, Ogelo said God saved him and gave him a "second chance".