In this photo taken from footage from Citizen TV, via the Kenya Defence Forces and made available Friday, Oct. 4 2013, a man reported to be Khatab Alkene, one of the four armed militants walking in a store at the Westgate Mall, during the four-day-long siege at the Westgate Mall in Nairobi, Kenya which killed more than 60 people last month. A Kenyan military spokesman has confirmed the names of four attackers as Abu Baara al-Sudani, Omar Nabhan, Khattab al-Kene and Umayr. (AP Photo/ Kenyan Defence forces via Citizen TV)

Kenyan authorities are cracking down on vendors who do not register people for cellphone lines, following revelations that the terrorists who attacked a Nairobi mall last month communicated using unregistered lines, officials said Tuesday

Al-Qaida-linked Somali militant group al-Shabab has claimed responsibility for the Sept. 21 terrorist attack that killed at least 67 people at the Westgate Mall in Nairobi. Kenya's military on Friday released images from closed-circuit television footage in the mall that appeared to show at least one of the suspected terrorist speaking on a cellphone.

Four chief executives of mobile phone companies have been questioned by investigating officers over their firms' registration of activated cellphone lines, said Kenya's Director of the Criminal Investigations Department Ndegwa Muhoro.

"We have arrested and prosecuted a number of vendors who were selling these SIM cards and we summoned the CEOs of these firms to explain what they have done to stop the crime," said Muhoro.

Police investigating the terror attack lack information on the people who bought the SIM cards and that information would have assisted with their investigations in to the attack, he said.

The chief executive officers of Safaricom, Telekom Kenya, Essar Telecom Kenya and Airtel Kenya later denied there are active SIM cards which are unregistered.

The mobile operators said that since the publication of SIM registration regulations, all four operators "have gone over and above what is required by law."

"We wish to inform the general public that since the publication of the SIM registration regulations in January of 2013, the mobile operators continue to maintain and update a registered subscriber base of over 30 million mobile phone users," the mobile operators said in a joint statement.

Vodafone PLC is a substantial shareholder in Safaricom, which is publicly listed. France Telecom has majority shareholding of Telekom Kenya, and Essar Telecom Kenya is owned by the Essar Group and run by the brand yuMobile in Kenya.

In 2010 Kenya made it mandatory for all SIM cards to be registered following an upsurge of phone-related crimes. At the time kidnappers were receiving ransoms through mobile money transfer services.