Kismayo, al-Shabaab's stronghold, is 80 miles north of the lodge where the British couple were staying. But Kenyan police described the perpetrators as "bandits," the BBC reported.
The Foreign Office warned of there being "a high threat of clashes between the Kenyan military and armed Somali groups along the border", citing recent attacks on Mandera by Somali militias; heavy fighting was reported over the weekend between al-Shabaab and Somalia's transitional federal government forces near the town. Casualties were confirmed on both sides as fighting continued to affect the border town of Burahashe.
Australia's foreign affairs ministry describes a "high risk" of kidnapping against westerners in Kenya and Somalia. A French yachtsman was killed by Somali pirates last week, while his wife was rescued by the Spanish navy. A Danish family of five, including three children, and two crew were released by pirates after being held seven months.
Kenyan police had been on high alert for the anniversary of the 9/11 attacks. Kenya had its own bitter experience of transnational terrorism in 1998 when al-Qaida bombed the US embassy in the capital Nairobi. The country's proximity to Somalia led it to being under scrutiny as prone to terrorism financing.
• This article was amended on 12 September 2011 to remove the suggestion that Lamu island had been racked by civil war, an error that was introduced at the editing stage. Guardian News
Kenyan police suspect Somali link after kidnap and murder of British couple