By Matthew Mpoke Bigg and Joseph Akwiri
NAIROBI/MOMBASA, Kenya (Reuters) - Police were tracking four suspected militants in Kenya's coastal tourist region on Thursday, broadening the investigation into a weekend attack on a Nairobi shopping mall that killed at least 72 people.
In Nairobi, experts from U.S., British, Israeli and other agencies have joined Kenyan officers investigating the Westgate mall where militants from the al Qaeda-aligned Somali group al Shabaab launched a well-planned assault on Saturday.
The high-impact attack has highlighted the reach of al Shabaab beyond Somalia, where Kenyan troops have joined other African forces, driving the group out of major urban areas, although it still controls swathes of the countryside.
"We have four suspects within Mombasa who we are closely watching. They came back to the country after training in Somalia," Robert Kitur, county police commander in the port city and beach tourist hub, told Reuters.
Western nations have long feared turmoil in Somalia was providing a training ground for militants. Kenya sent its troops into its northern neighbour in 2011 to fight the group it blamed for attacks along the border and kidnappings on the coast.
Highlighting the lawless nature of Kenya's northern region near Somalia, two policemen were killed on Thursday in an assault on a administrative post in that region. Al Shabaab said its fighters carried out this latest raid.
Part of the Westgate mall collapsed in the four-day siege that followed Saturday's attack, burying some bodies and hindering investigations, although forensic experts have started work even as the army continues to comb the building for explosives.
"The army are still in there with the forensic teams," said one senior police officer near the mall that was crowded during lunchtime on Saturday when attackers armed with assault rifles and grenades stormed in.
Officials say the death toll of 61 civilians, six members of the security forces and five militants is unlikely to rise much further, although some bodies of the attackers may be buried.
However, the Red Cross has said there were still 71 people listed as missing.
Several details of the attack remain uncertain: the final death toll, the identities of attackers, how many there were - officials indicate about a dozen - and how they staged such a sophisticated raid that lasted so long.
The attack has dented Kenya's image as a tourist destination, damaging a vital source of revenues.
But rating agency Moody's said that although the attack was "credit negative" it would not effect foreign direct investment or a planned Kenyan Eurbond later this year.
Police in Mombasa said they were tracking a network of suspects linked to al Shabaab, which said it launched the mall attack to demand that Kenyan troops quit Somalia.
"We want to let them be as free as possible, as we gather evidence and information on their activities before we arrest and charge them," Mombasa police commander Kitur said.
Another counter-terrorism officer, who asked not to be named, also said four suspects were being tracked and added that two well-armed suspected militants killed in an August operation could have been planning a similar attack in Mombasa.
"I will be surprised if they don't link the Nairobi attackers to those terrorists we killed in Mombasa," he added, without giving further details.
Many of Kenya's Muslims, making up roughly 10 percent of its 40 million people, live in the coastal region which also has a border with Somalia.
Experts on Somalia and the region have said that the porous border has allowed Kenyan sympathisers of al Shabaab to cross into Somalia for training.
"They are coming back because our armed forces destroyed their training ground there," said Kitur.
The coastal region also has been the target of attacks by a separatist movement, the Mombasa Republican Council, although that group has long denied it has connections with al Shabaab.
One regional diplomatic source said the international focus on rebuilding Somalia since al Shabaab retreated from urban areas may have distracted attention from the group's activities in Kenya and the possibility that it had built up cells there and in other nations.
He pointed to al Shabaab's last big attack, a double bombing in Uganda in 2010 that killed 77 people who were watching soccer. Local allied groups assisted in that, he said.
Several Western nations, including the United States and Britain, provide security training and other support to Kenya, considering it a vital regional ally in the fight against militant Islamism. (