Kenyan MPs have started an investigation into alleged intelligence failings over the deadly Westgate shopping centre attack.
The head of the parliament's defence committee says "people need to know the exact lapses in the security system".
There are reports the NIS intelligence agency issued warnings a year ago.
Some 67 people were killed and many injured after al-Shabab militants stormed the Westgate centre in the capital Nairobi on 21 September.
Kenya's Red Cross says the number of people still believed to be missing is 39, down from an earlier figure of 61.
Five militants were killed by the security forces during the four-day siege and 10 people have since been arrested, the authorities say.
Al-Shabab, a Somali Islamist group, said the attack was in retaliation for Kenya's military involvement in Somalia.
Security sources have told the BBC that the militants rented a shop there in the weeks leading up to the siege.
The parliamentary defence committee has started to discuss the procedures for their meeting with the security chiefs but the BBC's Robert Kiptoo at parliament says the actual questioning may not begin today.
The MPs are expected to visit Westgate later on Monday.
Committee head Ndung'u Gethenji had said the questioning of the security chiefs, including the head of the National Intelligence Service (NIS), Michael Gichangi, would begin on Monday.
He told the BBC last week 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.
Kenyan newspapers have reported that the NIS warned a year ago of the presence of suspected al-Shabab militants in the capital and that they were planning suicide attacks, including on the Westgate shopping centre.
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 had quoted counter-terrorism reports as saying.
A dossier from the NIS - 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 has reported 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 Interior Minister Joseph Ole Lenku, Treasury Minister Julius Rotich, Foreign Affairs Minister Amina Mohammed, Defence Minister Raychelle Omamo and Kenya Defence Forces chief Julius Karangi.
On Sunday, Mr Lenku refused to answer questions on the issue, saying the information was confidential and would not be discussed in public.
However, a senior interior ministry official earlier denied that ministers had ignored intelligence warnings.
The official - who was speaking on condition of anonymity - told the BBC the government received intelligence daily, that action was taken and that many attacks had been averted.