Four civilians have been killed by unidentified assailants who attacked a vehicle with rocket-propelled grenades and machine guns near the Kenya-Somali border, police and officials with Kenya's Red Cross say.
The attack occurred on Thursday in Mandera, in Kenya's sparsely populated northeast, close to the porous borders with Ethiopia and Somalia.
"The vehicle was carrying eight people but four of them have died," a police officer in the region said on condition of anonymity.
The vehicle was reportedly carrying education and security officials ferrying secondary school examination papers. It was not immediately clear who was behind the attack.
"We have just received reports that a grenade was hurled at a government vehicle, which was leaving Lafey and coming here to Mandera," a teacher at a Mandera school, who did not wish to be named, said.
Three other Mandera residents told the Reuters news agency they had heard reports of four deaths. Nelly Muluka-Oluoch, a Kenyan Red Cross spokeswoman, said agency personnel who went to the site of the attack confirmed the number of casualties.
Kenyan authorities have tightened security after Somalia's al-Shabab threatened to attack the country in retaliation against its military offensive against them.
Al Jazeera's Peter Greste, reporting from Nairobi, said: "The main suspicion is going to fall towards al-Shabab [but] I think we need to be very careful about linking these too quickly."
Nairobi was rocked this week by two grenade attacks that killed one person and wounded dozens. Kenyan security officials suspect al-Shabab, who have not claimed responsibility.
"Al-Shabab tends to attack much more symbolic targets, with much higher body counts," our correspondent said, "but, clearly, that link is one that the authorities are going to be looking at very closely."
A Kenyan suspect arrested after two grenade blasts earlier this week in Nairobi admitted in court that he was involved in one of the attacks, and claimed to be a member of al-Shabab.
Elgiva Bwire Oliacha, 28, also known as Mohammed Seif, pleaded guilty to nine charges, including involvement in Monday's explosion at a busy bus stop which left one person dead and several more injured.
An officer with the anti-terror police unit who brought Oliacha to court on Wednesday said authorities would bring more charges against him in court on Friday.
The officer, who wanted to remain anonymous, said that Oliacha was expected to plead guilty to all of those charges as well.
Source: Al Jazeera and agencies