The agency said 14 of the missing were found alive and seven bodies were identified in the morgue, the BBC reported Monday.
Before the latest information from Kenyan Red Cross, the Kenyan government said at least 67 people were killed after al-Shabaab militants stormed the Westgate mall Sept. 21.
Lawmakers have begun an investigation into alleged intelligence failings related to the attack.
The Red Cross said some relatives didn't alert the humanitarian group that people listed as unaccounted-for were no longer missing.
A Red Cross tracking manager told the BBC "some were reports from people who could not get through to their relatives on the phone and thought they might have been at the mall."
Five militants were killed during the four-day siege and 10 people have been arrested, authorities said.
Al-Shabaab, an Islamic militant group based in Somalia, said the attack was in retaliation for Kenya's military involvement in Somalia.
It has been reported the militants had rented a mall storefront in the weeks leading up to the siege.
The Daily Nation reported Kenyan intelligence established al-Shabaab leaders focused on multistory Westgate Premier Shopping Mall and the Holy Family Basilica as targets of an attack early this year.
Government leaders who reportedly received the intelligence briefings include Interior Minister Joseph Ole Lenku, Treasury Minister Julius Rotich, Foreign Affairs Minister Amina Mohammed, Defense Minister Raychelle Omamo and Kenya Defense Forces leader Julius Karangi.
Lenku Sunday refused to answer questions on the issue, citing confidentiality.
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