A pioneering British wildlife documentary-maker shot dead at her home in Kenya early this morning may have been killed for her conservation work.

Joan Wells Root, 69, was shot three times in her bed at her farmhouse on the shores of Lake Naivasha about 90 km (55 miles) northwest of Nairobi, police said.

Simon Kiragu, the Naivasha police commander, said that a guard had seen two gunmen breaking into Mrs Root's home at around 1am, local time. One man has been arrested in connection with the killing.

Mr Kiragu told The Times that early indications suggested that Mrs Root had been murdered, possibly for her conservation work on Lake Naivasha, the Rift Valley's only freshwater lake.

She was an outspoken critic of poaching and illegal fishing on the lake, where a section of the shore was named after her.

"Nothing was stolen from the house, the thugs just shot the deceased and left," said Mr Kiragu, adding that sniffer dogs were being used to track the killers.

Mr Kiragu said that police recovered seven AK-47 shell casings at the scene and that Mrs Root had bled to death. He said that police were questioning the guard and her other household staff.

Mrs Root, the daughter of a coffee farmer, grew up in colonial Kenya before becoming an influential photographer and filmmaker with her former husband, Alan. Together, the couple made a series of groundbreaking wildlife films, including the award-winning Journey of the Wildebeest in 1975.

The couple developed inventive filming techniques, attaching cameras to the shells of tortoises and burying them in the ground to film herds as they thundered overhead. They also used hot air balloons to film migrations and were the first people to fly a balloon over Mount Kilimanjaro. David Attenborough once said Alan Root was the man who made "natural-history filmmaking grow up".

Mike Limley, a producer and scientific adviser who worked alongside the Roots on several films, said today that Mrs Root was a fearless conservationist who had been "very active" in environmental work in Kenya.

"She was a fantastic lady, an elegant lady, This is a real loss," he said. "Anything she could do, any cause she could support, anything she do for conservation work, she would get involved."

"Some of the films the Roots made, their early work especially, was pioneering," said Mr Limley. "They worked so hard to get the shots, to get as close as they could. They used endoscopes to get into termite mounds."

"Joan was Alan's sidekick, there all the time. She would help make the sets, build things. She often filmed Alan while he was filming because we often needed that."

Referring to one incident that made Mrs Root famous for getting as close to her subjects as possible, Mr Limley added: "She was the one who would wear the mask so the cobra would spit in her eye." During a separate film, Mrs Root had her face mask bitten off as she filmed a hippopotamus underwater.

A spokesman for the BBC Natural History Unit said: "We were shocked and saddened to learn of Joan’s murder. Alongside her former husband Alan, Joan produced many wonderful films documenting African wildlife over many years."

"Joan’s death is a great loss to the wildlife film-making industry and to all those who champion the beauty and conservation of Africa’s wildlife. We extend sincere condolences to her family and friends."

In recent years, Mrs Root worked to protect Lake Naivasha. She kept waterbuck, dik diks, tortoises and antelope and even tried to raise a young hippo in her compound, according to local residents and interviews she gave.

Although a popular tourist destination, Naivasha is fraught with animosity between foreign investors, mainly flower farmers, and the local population over access to resources, and has been the scene of violent attacks on European and white residents.

John Goldson, the British owner of the Crater Lake Lodge in Naivasha, was shot and killed in July after he tried to free one of his security guards who had been taken hostage.

Lack of security in Kenya received international coverage in April when Tom Cholmondeley, the heir to the fifth Baron Delamere, shot dead a Masai wildlife warden who was conducting an undercover operation on his farm. Mr Cholmondeley said that he thought that the warden was a robber.

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