A UN investigator into extra-judicial killings in Kenya has called for an independent inquiry into the murder of two human rights activists.
Oscar Kamau Kingara was shot dead along with a colleague hours after a government spokesman accused their group of aiding a criminal gang.
A consortium of human rights groups in Kenya says it holds the government responsible for the death.
The police have denied any government involvement in the killings.
But UN investigator Philip Alston said that in the circumstances, suspicion was bound to fall on the Kenyan police.
"It is extremely troubling when those working to defend human rights in Kenya can be assassinated in broad daylight in the middle of Nairobi," said Mr Alston, who last week released a report accusing Kenya's police of involvement in extra-judicial killings.
Mr Kingara is one of those who gave evidence to Mr Alston.
"This constitutes a major threat to the rule of law, regardless of who might be responsible for the killings," said the UN investigator.
Mr Alston's report last week called for Kenya's top policeman and the attorney general to resign for failing to address police impunity.
Mr Kingara was shot dead in his car near State House, the official residence of the Kenyan president.
Afterwards university students clashed with riot police, with students refusing to release the blood-spattered car and the two bodies to police, according to a report on the Associated Press.
The killings have outraged the human rights community.
"The human rights community in Kenya holds the government fully and wholly responsible for the assassinations," Cyprian Nyamwamu of the Kenya Human Rights Consortium told the BBC.
"These two colleagues, members of the Oscar Foundation, are genuine, established, renowned human rights defenders," he said.
Mr Kingara's Oscar Foundation Free Legal Aid Clinic published a report last year, which said that 8,040 young Kenyans have been executed or tortured to death since 2002 in a police crackdown on a gang known as the Mungiki.
Earlier on Thursday, a government spokesman had accused the Oscar Foundation of raising money for the Mungiki.
Also on Thursday, members of the gang had led protests against police violence in the capital Nairobi and other towns.