A lawmaker who knew the geologist said Campbell Bridges, 72, had received death threats and Bridges' attackers were incited to kill him.
On Tuesday, about a dozen people attacked Bridges after he stopped to remove a log blocking a road, police said. Bridges had been driving to a mining camp by Tsavo West National Park, near where he lived and made one of his most precious discoveries of gemstones decades ago.
"We suspect those who killed him are probably employees of other people who want to mine in the area," said local Deputy Police Chief John Leshindoro.
Bridges was accompanied by his son, Bruce, and at least two colleagues during the attack. Police said his son was not hurt, but two colleagues were seriously injured.
The attackers "were screaming, 'We're going to kill you all,'" Bruce Bridges told the BBC.
Bruce Bridges said he and his father had been receiving death threats for three years, but police did nothing to stop it. He accused police of similarly ignoring their reports of illegal digging by bandits, including hours before Tuesday's ambush.
"My father and I had heard these bandits were digging trenches on our land, so we took that information to the police and asked them for extra security," Bruce said.
"They refused to aid us in any way and basically told us to go it alone. They left us to die," he said. "The family is destroyed - and we are angry. For this country to let what happened happen to him is a tragedy."
A Kenyan lawmaker, Johnstone Muthama, said Bridges told police about three death threats he had received - and had named the people who had issued them.
The local police chief, Herbert Khaemba, told The Associated Press that Bridges had told police once about the threats. He said officers did warn those people to stop threatening the geologist.
Khaemba said he also asked Kenya's Department of Mines and Geology to intervene, because those threatening Bridges were disputing his right to use a mining permit in the area. Bridges had been mining for gemstones in the vicinity of Tsavo since the early 1970s.
Muthama said Bridges' killing may have been motivated by anti-foreigner hysteria whipped up by political leaders.
"This is a problem caused by the leadership, who incite local people (by saying) that this is a foreigner who wants to take resources, so flush these people out," Muthama told The Associated Press.
Muthama, who owns a big gem mining business, said he had known Bridges for 25 years and together they founded the Kenya Chamber of Mines. Muthama is patron of that organization.
In the 1960s, Bridges discovered a rare green variety of garnet initially in modern-day Zimbabwe and later in Kenya's neighbor Tanzania. Today the green gemstone, mined chiefly in Tanzania and Kenya, is named tsavorite after the Tsavo national parks.
Bruce Bridges paid tribute to his father as "an amazing man" and "the world's most famous gemologist."
"Anyone in the industry would say it. No one had a resume like him," he told the BBC.
Southeast Kenya is sparsely populated and impoverished, because its dry, low-lying land is poor for most crops. The area's gemstone riches are mined by handful of companies. Many locals eke out their living on plantations growing sisal, a plant that produces stiff fibers for making rope.
