MOMBASA, Kenya (Reuters) - Four Kenyan policemen went on trial in Mombasa on Tuesday accused of killing the son of a British aristocrat in police custody in 2012, a high-profile case that has become a litmus test of accountability for Kenya’s security services.

Alexander Monson was found dead in his cell after being arrested for what the police said was smoking cannabis during a night out in Diani, near Mombasa on Kenya’s coast.

Naftali Chege, Charles Wangombe Munyiri, Baraka Bulima and John Pamba are accused of killing the 28-year-old, son of Nicholas, the 12th Baron Monson, and heir to a family estate in Lincolnshire, in eastern England. They have denied the charges.

Two reports by government pathologists, seen by Reuters, concluded that Alexander 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.

The police have said Monson died of an overdose, but his mother Hilary Monson told the court that her son had not been a drug addict at the time of his death.

“I know my son, like many youths, used drugs. He had a problem at one point, but he had gone through a great deal of suffering to sort himself out,” she said.

Hussein Khalid, director of the local rights group Haki Africa, said he was concerned that it had taken so long to bring the case to trial, but added:

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“All the same, it’s a good example for the country that finally people who were one way or another involved in the death of an innocent person while in police custody are now standing trial.”

Kenya’s police watchdog (IPOA), created in 2011 and funded by international donors keen to improve accountability, can investigate the police on its own initiative or after receiving a complaint from the public.

It has the power to order any serving or retired officer to appear before it. IPOA submits the findings of its investigations to prosecutors, who decide whether to pursue a criminal trial or order an inquest.

It has received thousands of complaints about brutality, extrajudicial killings and corruption. But successful prosecutions remain relatively rare.