Kenya's High Court on Monday jailed four policemen found guilty of the manslaughter of Alexander Monson, the son of a British aristocrat who was found dead in a police cell in the beach town of Diani in 2012.

Judge Eric Ogola issued the verdicts in the coastal city of Mombasa at the end of a high-profile case that has shone a spotlight on police brutality in the East African country.

"This should send a strong message to the Kenyan police force to have respect for human life," the victim's mother, Hilary Monson, told the court, fighting back tears.

The four policemen are Naftali Chege, Charles Wangombe Munyiri, Baraka Buluma and John Pamba. They all hung their heads after the verdict was announced, while one cried quietly.

Chege was sentenced to 15 years in jail, five of which were suspended. Munyiri was sentenced to 12 years, six of which were suspended. Buluma was given nine years, five of which were suspended, and Pamba was sentenced to 12 years, six of them suspended.

"I just feel like after waiting for 10 years, the sentence given to the accused persons isn't enough for a mother who lost her child in such a brutal manner," Monson's mother told journalists after the sentencing.

Monson, 28, was found dead after being arrested for what police said was smoking cannabis during a night out in Diani, near Mombasa on Kenya's Indian Ocean coast.

"The drugs were planted on the deceased after his death as a cover-up," said Ogola, adding that Monson was in perfect health before his arrival at the station and that he was "brutally tortured" while there.

"I am satisfied that the death of the deceased was caused through unlawful omission on the part of the accused persons for failing to seek medical care for the deceased in good time," Ogola said.

Monson was the son of Nicholas, the 12th Baron Monson, and heir to a family estate in Lincolnshire in eastern England. The elder Monson was also in the court room and nodded slowly as the verdicts were announced.

"I think the verdict could have been stronger, but I know obviously the suspects will face some punishment in jail," he said.

Two reports by government pathologists, seen by Reuters at the time, concluded that Monson had died after suffering a traumatic blow to the head. An inquest found there had been attempts to cover up the incident and threats against witnesses.

Kenyan police face frequent accusations of brutality and extrajudicial killings from civilians and rights groups, but officers are rarely charged and almost never convicted.

The Independent Policing Oversight Authority was established in 2011 to investigate police misconduct and has received millions of dollars in foreign funding.

Kenyans have filed thousands of complaints against police since its creation, but the organization has only secured 13 convictions against officers.