The heart of South Africa's corn belt boasts sprawling farms, cattle ranches, grain silos - and farmers packing assault rifles.

Targeted in a series of killings, white farmers have organized rapid-reaction forces, set up roadside checkpoints and linked themselves through an emergency radio network. They also have crossed the line into vigilantism, possibly even killing a suspect.President Nelson Mandela has ordered an investigation into the attacks on the whites, which some farmers claim are part of a black conspiracy to run them off the land. Authorities say robbery, not race, is the motive.

Mandela's chief investigator, Police Supterintendent Karel Brits, told The Associated Press that most of the last year's 290 attacks were motivated by robbery. Mpho Madevu, the mayor of Schweizer-Reneke and a member of Mandela's African National Congress, believes there is more to it.

Conditions for black farm workers have improved little since apartheid ended with the ANC's 1994 victory in the country's first all-race elections.

Farmhands in this north-central Afrikaner stronghold have no guaranteed minimum wage and complain of abuse.

"Many black workers are treated inhumanely, and some are even killed," said Madevu. "It's a slavery of a special type. Parents are beaten in front of their children.

"Vengeance becomes their character - they come back to that white farmer who abused him, or abused his parents, and settle things."

Annell Boonzaaier, a thin woman in a flower-print dress, says murder was in the hearts of four black men who came to her farmhouse on Nov. 30 when she and her husband Nico were away.

They pocketed her jewelry, took her husband's shotgun and pistol, and waited for their return. They shot her husband dead; she fled, running barefoot the mile to another farm and fracturing her right arm along the way. She doesn't recall breaking it.

Word of the attack spread on the radio network, and 100 pickups carrying heavily armed farmers converged on the Boonzaaier farm.

The farmers are army reservists and members of the Rural Safety Committee, created by the government last year and credited with reducing the number of farm killings from 109 in 1996 to 61 last year. Civil-defense officials, farm unions and police participate as well.

The farmers caught the suspects in a cornfield, beat them and turned them over to police. One suspect, Frank Masilo, grew up on a neighboring farm and briefly worked for the Boonzaaiers last year.

"They didn't do anything to conceal their identities," Boonzaaier said at the courthouse recently, waiting for a hearing in the case. "They came to rob and to kill."

After the judge postponed the case, Boonzaaier, the cast still on her arm, stood and stared into Masilo's face. Masilo averted his eyes.

Eleven days after Boonzaaier attack, Schalk Bruwer and his wife were shot in their general store about a mile away. They survived. Nothing was stolen.

"If we were extreme right wing, we could maybe understand it," said Bruwer, who didn't recognize the assailants. "But we've been doing business with the local black community for 19 years in this store. We didn't deserve this."