Kenya's government is facing questions over whether it ignored warnings about last week's siege on a shopping centre.

A leaked intelligence report obtained by Al Jazeera says security agencies were alerted about a possible attack on Westgate mall about a year ago.

According to the 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.

The report raises questions as to why security was not increased at the Westgate shopping centre, and adds to other credible reports that the attackers were able to rent areas in the mall to store weapons in preparation for the attack.

Many are also questioning the current investigation, which as yet has failed to account for dozens of people still reported missing.

Meanwhile, 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.

Kenyans have become increasingly frustrated over the government's unwillingness to share information about the attack. Almost no details have been released about what happened after the first hours of the siege.