Laos, run by an opaque Communist Party, ranks 160th out of 176 countries and territories in the corruption index published by Transparency International, a monitoring group. The authorities in neighboring countries say Laos has increasingly been used as a transit point for trafficked wildlife that is sent to consumers in East Asia, especially China and Vietnam.

Investigators believe that Mr. Vixay has enjoyed a great degree of protection from the Laotian authorities, and they point to voluminous evidence of Mr. Vixay’s wildlife-trafficking operations. A shipment of ivory and rhino horns intercepted by the Kenyan authorities in 2009 was addressed directly to Mr. Vixay’s company, Xaysavang Trading, in Laos.

The largest trove of evidence against Mr. Vixay, investigators say, came during the trial in South Africa of Chumlong Lemtongthai, a Thai national who South African prosecutors say was Mr. Vixay’s deputy.

In the trial, prosecutors laid bare a system in which Mr. Chumlong used Thai prostitutes to pose as rhino hunters, illegally using a loophole in South African law that allows hunters to bring back one horn as a trophy. Prosecutors called this “one of the biggest swindles in environmental crime history.” Mr. Chumlong was sentenced to 40 years in prison, which was reduced to 30 years on appeal.

Invoices presented as evidence in the trial showed that the rhino horns were in Mr. Vixay’s name and sent to his address in Laos. Mr. Vixay said in an interview that he had quit “a long time ago” what he described as his import-export business.

Yet Mr. Vixay’s wildlife-trading business is well known in the village along the Mekong River where his large walled compound is. During a visit by this reporter in February, a security guard who answered the door said the compound contained tigers, bears and other endangered animals whose trade is restricted or banned by a United Nations treaty. Villagers reported seeing regular truckloads of pangolins, an animal that resembles an anteater. Trading in pangolins is illegal under the United Nations treaty.

The United States government has been increasingly aggressive in combating wildlife trafficking, partly out of concern over the slaughter of elephants and rhinos for their tusks and horns. More than 800 rhinos were illegally killed in South Africa this year, far more than in any previous year.