Joan Wenn Root, 69, the daughter of a colonial-era British settler, was shot three times in her bed at her residence near the town of Naivasha, about 90 kilometers (55 miles) northwest of Nairobi, where she was raised and had lived for many years promoting environmental causes, they said.

Naivasha police commander Simon Kiragi said a guard had seen two thieves attempting to break into the home around 1:00 am (2200 GMT Thursday).

"Two thugs were spotted by the night guard, who then hit the alarm, prompting them to aim at the alarm and destroy it," he told reporters, adding that Root lived in the home alone while her ex-husband lives in an upscale Nairobi suburb.

Kiragi said police had recovered seven bullet shells at the scene that indicated Root had been killed with an AK-47 assault rifle and that a manhunt was underway to catch the perpetrators.

"We have sniffer dogs on the ground following trails," he said, adding that one of the dogs had been provided by a wealthy expatriate rancher in the Rift Valley.

Root was well-known in the Naivasha community for her efforts to conserve the environment and kept several species of animals, including waterbucks, dik diks and birds, on her compound, residents said.

Naivasha, a region rife with animosity between foreign investors, mainly flower farmers, and the local population over access to resources, has in the past been the scene of violent attacks on European residents although Root's murder is the first in several months.

At least three other Europeans have been slain in violent robberies in the Rift Valley since September 2004, prompting great unease and major security fears in the community.

Amid the concern, a prominent British aristocrat, Thomas Cholmondeley, son of the 5th Baron Delamere, shot and killed an undercover game warden on his Rift Valley ranch but escaped a murder trial, insisting he acted in self-defense.

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