By Dennis Okari for BBC
Possibility 1: Al-Shabab
Al-Shabab has said it carried out the attack in order to take revenge on Kenya for the presence of its troops in Somalia, where they are battling the militants, as well as for the killing of radical clerics linked to al-Shabab in the port city of Mombasa.
If the Somali group is to be believed, then it may have changed tactics for fear of losing support.
Many women and children, including Muslims, were among the 67 people killed during the siege of the Westgate shopping centre in Nairobi last September.
The indiscriminate attack angered some al-Shabab sympathisers, and the Mpeketoni attack could have been a way of sanitising the group’s image: kill the men, spare the women and children.
However, not all of the women were spared – at least 12 women whose husbands were not at home during the raids were abducted, local residents have told the BBC.
Sowing terror among ordinary Kenyans could be a strategy to increase pressure on the government to withdraw its forces from Somalia.
Possibility 2: Local dispute
Mpeketoni is a farming area, not a popular tourist resort like the nearby Lamu island, and the attack took the country and security agencies by surprise.
In the past, terror groups have concentrated on Kenya’s major towns and cities, or targeted foreign tourists in order to gain maximum international publicity.
According to reliable accounts, the attackers were well organised, and as soon as they finished their mission, they disappeared, supporting the theory that they may be locals.
“This… was not an al-Shabab attack. Evidence indicates that local political networks were involved in the planning and execution of a heinous crime,” said President Kenyatta.
“The attack in Lamu was well planned, orchestrated and politically motivated ethnic violence against a Kenyan community, with the intention of profiling and evicting them for political reasons,” he said.
Many of those who died in the attack came from Mr Kenyatta’s Kikuyu community.
There are long-standing political and ethnic divisions in this area.
It could be that local Somalis and Oromos who claim the area as their ancestral home are trying to drive out Kikuyus, who they see as interlopers.
The president’s father, independent Kenya’s founding President Jomo Kenyatta, gave the area to ethnic Kikuyus in the 1960s.
Such disputes over land ownership were behind much of the ethnic violence which broke out across Kenya after the disputed 2007 elections.
A group of Kenyan Somalis or Oromos could easily wave al-Shabab flags and shout slogans such as Allahu Akbar (God is great) in order to divert blame.
Possibility 3: Separatist rebels
President Kenyatta did not name the local political group he was accusing.
In recent years, the Mombasa Republican Council (MRC) has been campaigning for autonomy for Kenya’s largely Muslim coastal region, arguing that local people see little economic benefit from the region’s trading ports and tourist industry.
It has been accused of carrying out small-scale attacks in and around Mombasa, which it has strongly denied.
But it might be that it has launched an armed insurrection.
Possibility 4: An alliance
Of course, it could be that one of the Kenyan groups has decided to work with al-Shabab, which would explain some of the confusion.
President Kenyatta would want to downplay the al-Shabab angle in order to try and protect Kenya’s embattled tourist industry, so if there were an alliance, he would focus on the local group.
This would also enable him to send Kenya’s security services after some of his political enemies.
While if some al-Shabab fighters were involved, it would enable the group’s spokesman to say they were behind the attack, even if it was not solely their idea.
They have never previously said they carried out an attack which later proved to be untrue.
And al-Shabab might like to target ethnic Kikuyus in order to take their battle right to the president’s doorstep.