Kenyan government ministers received warnings of a possible attack before al-Shabab gunmen stormed the Westgate shopping centre, reports say.
Leaked intelligence briefings detailed plans for attacks in Nairobi and Mombasa, according to Kenyan media.
Four cabinet ministers and the head of Kenya's defence forces are said to have been informed.
At least 67 people were killed after al-Shabab stormed the mall on 21 September.
Briefings were given to the ministers "informing them of increasing threat of terrorism and of plans to launch simultaneous attacks in Nairobi and Mombasa around September 13 and 20, 2013", Kenya's Daily Nation quotes counter-terrorism reports as saying.
The warnings were first made in January, according to the newspaper, and again from the beginning of September.
A dossier from the National Intelligence Service - amounting to more than 8,000 pages according to Kenya's Standard newspaper - also suggests the Israelis issued warnings that buildings owned by its citizens could be attacked between 4 and 28 September.
Westgate is partly Israeli-owned.
The Daily Nation reports that Kenyan intelligence had established that al-Shabab leaders had begun singling out Westgate and the Holy Family Basilica for attack early this year.
Government figures said to have received the intelligence briefings include Treasury Minister Julius Rotich, Interior Minister Joseph Ole Lenku, Foreign Affairs Minister Amina Mohammed, Defence Minister Raychelle Omamo and Kenya Defence Forces chief Julius Karangi.
The head of the National Intelligence Service, Michael Gichangi, is due to be questioned by Kenyan Members of Parliament on Monday.
An unnamed senior intelligence official told the AFP news agency that the government had been casual in its approach to the briefings it received.
"There is no way one can say there was no intelligence on this attack because those reports started trickling in from late last year. And they were specific with targets including Westgate."
The head of the parliamentary defence committee, Ndung'u Gethenji, told the BBC on Friday that "people need to know the exact lapses in the security system that possibly allowed this event to take place".
He also said they needed to understand "the anatomy of the entire rescue operation" amid allegations of confusion over who was in charge.
Funerals for some of those who died in the four-day long siege have continued after three days of official mourning came to an end on Friday.
The official death toll remains 67, but the Kenyan Red Cross maintains 61 people are still missing.
The UK Foreign Office has meanwhile confirmed that a sixth British citizen is now known to have died in the siege.
Al-Shabab, a Somali Islamist group, said it carried out the attack on the upmarket mall in retaliation for Kenya's military involvement in Somalia.
Security sources have told the BBC that the militants hired a shop there in the weeks leading up to the siege.
This gave them access to service lifts at Westgate enabling them to stockpile weapons and ammunition. Having pre-positioned weapons they were able to re-arm quickly and repel the security forces.
The BBC has also confirmed more details about how they executed their attack, with two vehicles dropping the Islamist extremists outside before they forced their way into the mall, according to sources.
They are also believed to have set up a base using a ventilation shaft as a hiding place, on the first floor.