PHNOM PENH (Reuters) - A Cambodian court on Thursday jailed a man for life for killing prominent government critic Kem Ley, in a murder case that raised suspicions of a political motive though the convicted killer testified that it was over a debt.

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Kem Ley, 46, was shot in Phnom Penh in broad daylight last year and his death drew tens of thousands of protesters onto the streets against a backdrop of growing political tension with veteran Prime Minister Hun Sen gearing up for elections.

Chuop Somlap, 45, a former Khmer Rouge soldier and Buddhist monk whose name means “meet to kill”, said during his trial that he had killed Kem Ley over a $3,000 debt, a statement disputed by the victim’s family and human rights groups.

“The accused did really kill Kem Ley,” judge Leang Samnath said, delivering the verdict against Chuop Somlap, who is also known as Oeuth Ang. “The killing was premeditated and the accused carried a weapon without permission.”

Human rights groups and Kem Ley’s supporters remain skeptical of the motive. The family said the activist did not owe any money.

Kingsley Abbott, the senior international legal advisor at the International Commission of Jurists human rights group, called for the investigation to be reopened.

“Until there is an independent, impartial and effective investigation to establish whether anyone else was involved in the killing, the victims of this terrible crime, including Kem Ley’s wife and children, will be unable to obtain justice,” said Abbott, who observed the trial.

The investigation had not established whether Chuop Somlap had the means to lend the money to Kem Ley or to purchase a handgun, he said, adding that it had also not drawn on footage from all the CCTV cameras near the crime scene.

Kem Ley founded an advocacy group called “Khmer for Khmer” and had been a frequent critic of Hun Sen, who opponents accuse of strong-arm tactics ahead of local elections this year and a general election next year.

Chup Somlap said during his trial that he had met Kem Ley once, a year before the crime, through a friend in Thailand. The critic had promised him a job and a house worth $20,000 if he gave him $3,000. He said he had reacted out of anger when he got nothing in return for his money.