Kenya's parliament is due to hold a hearing into last week's siege of a Nairobi shopping centre, as the International Red Cross says 39 people remain unaccounted for since the attack.

The announcement of the hearing came as a leaked intelligence report obtained by Al Jazeera showed that security agencies had been alerted to the possibility of the attack on the Westgate mall a year before the siege began on September 21.

The chairman of the parliament's defence committee told Al Jazeera on Monday that it was too early to judge the leaked report, but that the testimonies from senior security and intelligence officials this week should provide some answers.

The questions the government is facing also come as the International Red Cross released a report on Monday claiming that there are still 39 people unaccounted for since the siege.

Al Jazeera's Andrew Simmons, reporting from Nairobi, said that the Kenyan government has remained insistent that there remain no bodies of hostages among the rubble inside the shopping centre.

Simmons said that questions also remain about the attackers, whom the government insists could not have escaped through a tunnel that led several hundred metres away from the mall.

"The interior minister said in a press conference that there was no way [the attackers could have escaped]," said Simmons, "but in an interview shortly after with a Kenyan TV station, he said that defence forces did not have a blueprint of the shopping centre until Monday, whereas the attackers had access to a map and a shop within the mall weeks in advance."

Leaked report

According to the leaked report, officers were warned that al-Shabab was planning suicide attacks on the Westgate shopping centre and the Holy Family Basilica, a church in Nairobi, in September last year.

In January this year, the report looked at the possibility of attackers storming a building and holding hostages, much like the siege last week.

Earlier this month, the Israeli embassy in Nairobi raised concerns about a possible attack on Israeli citizens during the Jewish Holiday period in September, and also revealed that a number of Somalia-based al-Shabab fighters were apparently given refugee cards to enter Kenya.

Al Jazeera's Nazanine Moshiri, reporting from Nairobi, said the end of the report is also quite damaging to the Kenyan government as it lists recipients of the intelligence document, which includes top cabinet officials.

Meanwhile, Kenyan security services made another arrest on Sunday. Kenya has arrested 12 people since the attack but three have been freed, Interior Minister Joseph Ole Lenku said during a press conference.

He declined to say if any of those arrested had been in the mall during the attack.

Investigators have also identified a car used by the gunmen and found in it "an assortment of illegal weapons," said Lenku.