Relatives of U.S. military members missing in Vietnam are urging President Obama to press Hanoi on the whereabouts of anyone who may have been killed or captured during the post-World War II conflict.

Vietnam hopes that Obama will agree to lift an arms export embargo so it can better deal with China in the South China Sea dispute, while rights activists want him to hold to account a repressive one-party state seen as treating its critics abysmally during his visit on Monday.

According to The Telegraph, some Americans believe there is still some “unfinished business” to take care of in discovering the unknown fates of more than 1,600 military members who never returned home from the Vietnam War. Relatives of those military members want Vietnam’s help in accounting for those who may have died after being shot down or died as a prisoner of war.

Some others want Obama to seek answers as to whether Vietnam held Americans as prisoners of war after 1973 instead of releasing all the captives as the peace agreement called for. Hanoi has repeatedly insisted that it has provided Washington with its help in discovering the whereabouts of all of its missing personnel and denied that it held prisoners of war after the conflict was over. Just over 1,000 Americans have already been accounted for and had their remains returned, The Telegraph reported.

Even though the last 591 American POWs returned to the U.S. in April 1973, there are still suspicions that the then-North Vietnamese retained some prisoners of war to leverage an aid package with President Richard Nixon that could’ve enticed the end of the war. The theory states that the aid payment failed because of Nixon’s Watergate scandal and the North Vietnamese kept the prisoners.

The theory continues as saying that some American prisoners of war are believed to have been killed or to provide military secrets in exchange for their lives under the Communist rule, according to The Telegraph.

Speculation was also fueled by unconfirmed sightings of American POWs in Vietnam by a Vietnam expat and the findings of a 1993 Senate committee, headed by Secretary of State John Kerry. However, no real proof has ever been offered.

“We acknowledge that there is no proof that US POWs survived, but neither is there proof that all of those who did not return had died,” it concluded. “There is evidence, moreover, that indicates the possibility of survival, at least for a small number, after Operation Homecoming.”

Sen. John McCain, who was a prisoner of war captured by Vietnam, said he received “full access” to the Vietnamese records in the 1990s and didn’t believe there were any surviving American POWs remaining in the Asian nation.

Lt. Morgan Donahue’s plane was shot down over modern-day Laos in 1968 and has never been accounted for, his brother Jeffrey told The Telegraph. Jeffrey Donahue said he believes he was taken by the Vietnamese and was eliminated when they didn’t receive their aid from the Nixon administration.

“The government says he is dead. But I’m absolutely convinced he was still alive at the end of the war. I don’t hold any hope that he is alive now.”

The prisoners of war issue may not be on the series of topics in Obama’s visit to Hanoi. He’s expected to address the Communist nation on China’s threat and the country’s civil rights issues.

Ahead of Obama's visit, in what was seen as a goodwill gesture, Vietnam granted early release from prison to a prominent dissident Catholic priest. The Rev. Nguyen Van Ly has served several long terms in prison or been under house arrest for promoting political and religious freedoms.

Both Washington and international rights groups criticize Vietnam for jailing people who peacefully express their views by using vaguely worded security laws. Hanoi says that only lawbreakers are punished. In March, seven bloggers and activists were sentenced for "abusing democratic freedoms" and "spreading anti-state propaganda."

The Associated Press contributed to this report.

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