MONROVIA (AFP) - Liberian President Ellen Johnson Sirleaf vowed Tuesday to bring the killers of one of her political aides, Keith Jubah, to justice.

Jubah, the head of Liberia's Public Procurement and Concession Commission (PPCC), was shot as he returned to his home in Kakata, about 20 kilometres (12 miles) from Monrovia. He was a close confidant of the president and an influential member of the ruling Unity Party.

Investigators have so far been unable to establish a motive for the attack.

Sirleaf described him as "one of Liberia?s most decent public servants."

"I will ensure that those who killed Jubah will not go unpunished", Sirleaf said.

Keith Jubah's son, Prince, told AFP on Tuesday that he believed his father was a close friend of his killer.

Prince Jubah said his father had told him about receiving death threats from a friend over a disputed sale of land.