Brazilian police are investigating whether the fatal shooting of three rural activists was linked to their effort to win rights to land also contested by owners of a sugar mill.

The activists were shot on Saturday as they got out of a car near a landless workers' camp in the south-western Minas Gerais state.

A five-year-old girl, the granddaughter of two of those who were killed, survived the attack. No one has been arrested, a police spokesman said.

Watchdog groups said police were questioning land activists about the possibility the killings could have resulted from an internal conflict within their movement. The groups rejected that idea and accused landowners of paying gunmen to shoot the activists.

Carlos Calazans, head of the Minas Gerais branch of the federal department of land reform, known as Incra, said police were looking into the land dispute as a possible motive.

"It's definitely one of the theories for the motive behind this barbarous crime," he said. "I've no doubt these activists were summarily executed. But police have to follow all leads until they find the truth."

Calazans said the killed couple approached Incra last year seeking support in various land conflicts in the region, including the one with the mill owners. He said Incra tried to get the owners and activists to agree on the issue a few weeks ago, but the effort was unsuccessful.

Killings over land in Brazil are common, and people rarely face trial for the crimes.

The watchdog group Catholic Land Pastoral says more than 1,150 rural activists have been murdered in Brazil over the past 20 years. The killings are mostly carried out by gunmen hired by loggers, ranchers and farmers to silence protests over illegal logging and land rights, it says. Most of the killings happen in the Amazon region.

Fewer than 100 cases have gone to court since 1988, Catholic Land Pastoral says. About 80 of the hired gunmen have been convicted, while 15 of the men who hired them were found guilty, and only one is currently in prison.

According to Incra, those killed on Saturday were Clestina Leonor Sales Nunes, 48; Milton Santos Nunes da Silva, 52; and Valdir Dias Ferreira, 39.

The girl was apparently the only witness to the killings, which were carried out along a highway near the camp, about 25 miles (40km) south-east of Uberlandia. She told police a car cut off the one she was riding in with the victims, forcing it to stop. Either one or two gunmen then opened fire.

A statement on the Catholic Land Pastoral's website described the three victims as state leaders of the Landless Liberation Movement, one of several rural activist groups that invade land and set up camp, living on what they say is unproductive ground.

Brazil's agrarian reform laws allow the government to seize fallow farmland and distribute it to landless farmers. Nearly 50% of arable land belongs to 1% of the population, according to the government's statistics agency.

The latest killings come just before the month that landless worker movements typically step up invasions of what they say is unused land. The seizures are meant to mark the April 1996 killing of 19 landless activists in Para state.