Tokyo ? A Japanese team that returned Monday from a mission to investigate the United States’ first confirmed case of mad cow disease warned that American and Canadian cows still were vulnerable to an outbreak of the illness.

Japan, the world’s largest customer for U.S. beef, banned imports from the United States last month after the mad cow case was discovered. Canadian beef was banned seven months earlier when a case of the illness was detected there.

Both Washington and Ottawa are pressing Japan to drop the bans, arguing that their beef products are safe, but the findings of the 11-day Japanese mission to the United States and Canada advised caution.

The cow discovered in Washington state with bovine spongiform encephalopathy, or BSE, as the disease is formally known, was imported from Canada.

“It cannot be guaranteed that there will not be a recurrence of BSE in the United States,” the five-member team said in its report.

The report cited the close links between the two North American countries’ beef industries. The United States imposed restrictions on Canadian cattle and beef after the case was discovered there in May.

Mad cow disease is a public health concern because scientists believe humans who eat brain or spinal matter from an infected cow can develop variant Creutzfeldt-Jakob disease. The incurable disease was blamed for 143 deaths in Britain during a mad cow disease outbreak in the 1980s.

That concern prompted several countries to close their borders to American beef last month.

Before the ban, Japan was the top importer of U.S. beef, snapping up $1 billion worth of American beef and beef products a year. It also imported about $55 million worth of Canadian beef in 2002 before halting trade in May.

The sharp drop in beef supplies has sent the price of both domestic and imported beef soaring in Japan. The Agriculture Ministry said Monday that retail prices reached a record high last week since it began monitoring such data in August.

Tokyo has scrambled to make up for the shortfalls, sending delegations to try to secure more beef from Australia and New Zealand. Neither country has reported a case of mad cow disease.