Japanese scientists met with U.S. agriculture officials Monday about the investigation into mad cow disease (search), as more cows from a Washington state dairy farm were killed.

Agriculture officials are killing 129 cows from the farm in Mabton with ties to a Holstein (search) that had the disease. Nine cows were euthanized Saturday and 22 on Monday, said Nolan Lemon, spokesman for the U.S. Department of Agriculture (search). He said between 30 and 40 cows will be killed each day beginning Tuesday until the process is complete.

Investigators determined that nine of the 129 cows came from the same Canadian farm where the sick Holstein was born, but could not rule out the other animals in the herd.

Meanwhile, a team of agriculture and health officials from Japan, one of more than 30 nations that have banned U.S. beef, met in Yakima with their American counterparts to discuss the mad cow case. The delegation was not made available to reporters.

Japan, the largest importer of U.S. beef, requires domestic cows to be tested before going to market and has urged the United States to tighten its testing procedures as a condition for a possible lifting of the ban.

The sick Holstein, slaughtered Dec. 9, had entered the country in a herd of 81 cattle from Alberta in 2001.

Investigators so far have traced a total of 10 of those cows to two Washington state farms. Neither farm was named. Both remained on a state hold order, similar to a quarantine, but no decision had been made about their herds, Lemon said.

Agricultural officials last week killed a herd of 449 calves, including an offspring of the infected Holstein, but said the calves were not tested for mad cow disease, in part because they were too young for the disease to be flagged on tests.

The cows killed this week will be tested for the incurable disease, called bovine spongiform encephalopathy, which eats holes in the brains of cattle. Humans can develop a brain-wasting illness, variant Creutzfeldt-Jakob disease, from consuming contaminated beef products.