Kenya, as many Kenyans would attest, has a reputation for official impunity, particularly its notoriously brutal police force.
A series of prominent murders, both of black and white individuals, remain unsolved with some cases stretching back decades.
Few, therefore, expected such a ruling, least of all Lord Monson, who admitted to feeling "complete shock" when he received the news.
"I was so prepared for an exoneration of the police," he said. "Kenya is notorious for its corruption and this is a quite extraordinary ruling.
"It's an extraordinary ruling not in as much as it is surprising in regards to what happened but that the judiciary are prepared to take on the police."
Alexander Monson was detained by police officers in the early hours of May 19, 2012 after he and a friend were stopped outside a nightclub on suspicion of smoking cannabis.
The friend was later released but Monson was arrested after allegedly being found with half a joint of cannabis in his pocket.
A family friend who subsequently went to the police station discovered him barely conscious on the floor of his cell. He later died in hospital, handcuffed to his bed, with his mother Hilary, who was born and raised in Kenya, by his side.
The police doggedly stuck to their version of events: Alexander Monson, they said, was a drug addict who suffered an overdose. They tried to revive him; they even took him to hospital. The bruises on his head were due to a fall. Those on his groin were not from a repeated kicking but were the result of "oral sex".
The ruling, with its rare rejection of police testimony, will raise hopes that other infamous cases could perhaps be resolved, among them the murder of Julie Ward, a British tourist killed in the Maasai Mara in 1988. Her father, John Ward, fought an unsuccessful and expensive campaign for decades to discover how she died, alleging that powerful political figures may have been involved in her suspected rape and murder.
It is unclear why the magistrate, Richard Odenyo, chose to withstand suspected pressure to rule the way he did. Even as Kenya has been accused of retreating into authoritarianism in recent years, the country's judiciary is increasingly independent.
Last year, the supreme court became the first in Africa to overturn a presidential election, ruling that a victory won by the incumbent president, Uhuru Kenyatta, was marred by "irregularities and illegalities".
Kenyatta went on to win a rerun, boycotted by his opposition challenger, but the ruling was seen as emboldening other judges.
Lord Monson suggested that Thursday's ruling may also have been triggered by disgust within Kenya's establishment against the police after a prominent human rights lawyer, Willy Kimani, was murdered in 2016 while in police custody.
