During a water festival last April, two Americans traveled by boat across the Mekong River from Thailand to Laos. They never returned.

The men, Michael Vang of Fresno, Calif., and Houa Ly of Appleton, Wis., are--or were--naturalized Americans of Hmong origin. They and their people came to the United States as refugees from the mountains of Laos, where the Hmong fought during the Vietnam war alongside American covert agents, then fled en masse after the communist victory in 1975.

Now, the families of Vang, 36, and Ly, 56, fear that the Laotian government may have sought to settle old scores by capturing and executing two middle-aged tourists apparently on a pleasure trip to their homeland.

The disappearance of the two men has become a cause celebre in the Hmong American community, and the U.S. government's response suggests that a new phase has begun in the long-troubled relationship between the 250,000-plus Hmong in America and their adopted country.

This month, the House of Representatives passed a resolution condemning the human rights record of Laos, threatening economic sanctions if the Laotian government does not cooperate in seeking the missing men, and urging the State Department to share more information with their families. An FBI investigation is underway, and the Defense Intelligence Agency is involved too.

Still, some legislators worry that the United States is not doing enough to find the two men.

"I personally don't believe the State Department has been aggressive enough in trying to find out what happened to these men, and they deserve better," said Rep. Mark Green (R-Wis.), whose district includes many Hmong, including the Ly family. "The Hmong won their citizenship through real hardship, and we want to make sure it means something."

America's main leverage is economic. Laos is one of just eight countries (including Iraq, Cuba and Yugoslavia) that still do not have normal trade relations with the United States, and the State Department has been seeking to change that since signing a bilateral trade agreement in 1997. But Congress has not obliged.

"It's very likely that Lao government officials are responsible for the kidnap and murder of these folks," said Rep. George Radanovich (R-Calif.), who represents Michael Vang's district and a large Hmong population in Fresno. "And I think the Lao government has to understand that it is inconceivable to normalize trade relations with them when they have our two Americans."

Landlocked and poor, Laos has been eager to do more business with U.S. companies. But with the Hmong and their advocates lobbying hard against normalized trade, the legislation has died--and that was before the disappearance of Vang and Ly.

"The Hmong are definitely emerging as a political force," said Philip Smith, Washington director for the Lao Veterans of America, which represents 12,000 Hmong Americans who fought with the Central Intelligence Agency during the Vietnam era.

"They're starting to vote, to write letters to congressmen in large numbers, to have demonstrations," he said. "In the past, the Hmong were basically ignored because people didn't know who they were and what they did for the United States, since it was all covert. But there are many signs that's changing."

For its part, the Laotian government has said it knows nothing about the two men. Like so many past mysteries along the Mekong, their disappearance is long on ramifications but short on facts.

According to two other Hmong Americans who were traveling with Vang and Ly, they went shopping on the afternoon of April 19 in the Thai town of Chieng Kham. There, the group was told by some Laotian officials that the border was open that day because of the water festival.

Some members of the group weren't interested and drifted away. But Vang and Ly were last seen boarding a boat with the Laotian officials.

The State Department says the disappearance wasn't reported until May 4. Later that week, according to relatives of Vang and Ly, the families received a phone call from an American consular affairs officer in Thailand reporting that Ly had been killed and that Vang, the younger man, was imprisoned in the Laotian capital of Vientiane.

State Department officials have since said the report was one of several conflicting stories they have been unable to verify, even though FBI agents twice have traveled to the area to investigate. They also said they were "dissatisfied" with the cooperation from the Laotian government.

Complicating the case is the Cold War pall that still hangs over relations between Laos and the United States, and especially between the Hmong and the Lao People's Democratic Republic, the official name of the country, which still has a one-party, communist leadership.

Ever since the CIA began parachuting agents into the hills of Laos in the early 1960s to recruit a secret army to fight the North Vietnamese and Laotian communists, the fate of the Hmong has been tied to the Cold War. While this seldom served the Hmong well--they suffered an epic defeat and a quarter-century of difficult diaspora--their leaders have been reluctant to let go of the conflict.

So while American policy regarding Laos has gradually become less confrontational, some Hmong remaining in Laos have continued to fight the government, and many Hmong Americans have been eager to support them.

Congressional staff members say they have received reports suggesting that Vang and Ly were resistance fighters crossing into Laos as infiltrators. Although no evidence has emerged to support this, the rumors have some currency, because Ly did serve during the Vietnam War--family members said he helped rescue American pilots downed in the mountains of Laos--and because Michael Vang belongs to the same Hmong clan as Vang Pao, the legendary Hmong general who has led his people for decades.

Congressional sources also said FBI investigators in Laos and Thailand have been told that the two men were carrying large amounts of cash when they disappeared.

Michael Vang's wife, Susie, said she had heard these rumors as well and dismissed them as "disinformation" spread by the Laotian government and, possibly, the State Department against her husband, who worked as a supervisor of a reforestation crew for the U.S. Forest Service.

Many Hmong are suspicious of the State Department, which they believe has worked against their interests in recent years. (The State Department, while acknowledging the tragic recent history of the Hmong, says there is no evidence that they are being persecuted in Laos today). A further irritant is the unexplained disappearance of another Hmong man several years ago, an important leader named Vue Mai.

In the early 1990s, the United Nations High Commissioner for Refugees, the United States, Laos and Thailand agreed on a program to close Hmong refugee camps in Thailand and send some 25,000 people back to Laos. It was a controversial plan in the Hmong community, and many refugees didn't want to return.

But Vue Mai, who had been a Hmong leader during the secret war and had served as the commander of the largest refugee camp, agreed in 1993 to return to Laos and to lead many of his people back. Within months of his return to Vientiane, Vue Mai disappeared. The Laotian government claimed no knowledge of his whereabouts, although his followers said he had been under constant surveillance.

"At that time, Hmong asked for help from many people in government but received very little," said Wangyee Vang, president of Lao Veterans of America. Since then, he added, the community has "worked very hard to talk to congressmen and to educate their staffs" about Laos and the Hmong. "If we had not done that, I don't think we would be getting the much better help we are getting today," he said.

There are, however, clear limits to this Hmong influence. For years Hmong Americans have protested the off-again, on-again repatriation of refugees in Thailand back to Laos. And in light of the House resolution condemning human rights in Laos, many thought the repatriations would be stopped.

But according to the United Nations, 254 Laotian refugees were sent back on Thanksgiving Day from the last official camp, known as Ban Napho. The remaining 139 are scheduled to be returned to Laos next month. As in the past, the Laotian government has guaranteed their safety.

Staff writer Steven Mufson contributed to this report.