The Islamist militants who stormed the Westgate shopping mall in the Kenyan capital of Nairobi are believed to have rented a shop in the complex to gain full access to the Israeli-owned plaza.

The al-Shabaab-backed militants were able to closely study the shopping centre as tenants, a move reflected in their well-planned operation, according to Kenyan security forces.

The group said that it targeted the mall because it was home to a number of Jewish and American premises.

Police and witnesses said the insurgents appeared to possess full knowledge of the layout of the mall.

The Kenyan authorities were following this line of investigation, but said it would take longer to confirm the reports.

"That is one of the theories we are pursuing, because there are claims some were tenants of one of the shops," a senior police official told the Standard Digital.

Investigators would be interviewing all tenants in the shopping centre as part of their inquirty.

The possibility of militants renting shops was discussed during Kenya's National Security Advisory Committee meeting while the army-led operation took place.

The venue is frequently visited by Westerners and high-profile Kenyans. The six-year-old shopping complex is usually busy during weekends, making it an attractive target for the militants.

The assailants as well as the victims belong to different nationalities. "A multinational collection from all over the world" descended on the Kenyan mall to wreak havoc, said the country's interior minister, Joseph Ole Lenku.

It has also emerged that there was a plot to launch a terror strike on the mall on an earlier occasion.

In July 2012, two Iranians carrying explosives were arrested. They were allegedly planning an attack on the shopping centre, targeting Americans, Britons and Israelis.