PREAH SIHANOUK PROVINCE, Cambodia—They were some of the most searing images of violence to emerge from the land crisis that swept Cambodia over the last decade. In scorching dry-season heat, the military shot villagers and torched their homes across this seaside province.

Witnesses said one eviction in April of 2007 at the village known as Spean Ches was particularly brutal. Security forces inflicted gunshot wounds at close range, used live fire to disperse crowds, and beat villagers, sometimes with batons that deliver electric shocks.

A joint force of about 150 members drawn from the police, the army, and the Royal Gendarmerie burned 80 houses and demolished another 26 homes.

“They used a type of fire gun to shoot flames to burn down the houses,” said Yeang Ren, 32.

Gendarmes arrested villagers, forced them to lie face down, and repeatedly kicked them in their heads, Ren said. “We felt great distress when we heard our houses being knocked down with an excavator.”

That month, a Cambodian navy unit burst into another community 15 miles away, beating one villager unconscious and burning down five houses to seize land that residents now say forms part of the campus of a local training school for the navy.

As property values rose over the last decade, Cambodia’s poorer rural and urban communities found themselves locked in land battles with the country’s oligarchy, which claimed rights to prime real estate. By 2014, more than half a million people had been affected by evictions, according to the Cambodian League for the Promotion of Defense of Human Rights, a human rights organization known as LICADHO, for its acronym in French.

The scale and violence of evictions at Spean Ches quickly became emblematic of the crisis, drawing the attention of the U.S. Embassy in its 2007 annual report on human rights. And in 2014, the Spean Ches eviction helped form the basis of a private legal action alleging crimes against humanity that was brought before the International Criminal Court.

Yet as the crisis unfolded, Washington intensified its relations with the Royal Cambodian Armed Forces and police.

Congress, in 1997, had outlawed assistance to foreign security forces known to have committed gross violations of human rights. Yet diplomatic files published by WikiLeaks and compiled by the nonprofit investigative journalism group 100Reporters show that American officials overlooked such violations in vetting Cambodian police and military personnel for their eligibility to receive U.S.-funded training—in some cases apparently in violation of the law.

Two years after the violence at Spean Ches, the U.S. Embassy in Phnom Penh recommended Colonel Seng Phok, a deputy commander in the Royal Gendarmerie, for U.S. training, the same man a human rights worker said was among the commanding officers during the eviction.

100Reporters also found that the United States provided training in investigative techniques to senior members of Cambodia’s National Police Commissariat who at the time were the subject of detailed murder and kidnapping allegations.

Those allegations were contained in court records and United Nations’ files that would have been easily accessible to the U.S. Embassy, which submitted their names to Washington for approval.

In a data-driven investigation carried out over more than a year using the “cablegate” cache of American diplomatic records, 100Reporters developed a navigable database of nearly 60,000 individuals from 129 countries selected for training by nearly 140 U.S. federal agencies.

Suspected narcotraffickers, killers, torturers, and foreign units involved in systematic extrajudicial killings were all found in the database. Many managed to pass through American vetting and receive training.

Ny Chakriya, chief monitor at the U.S.-funded Cambodian Human Rights and Development Association (ADHOC), which began assisting the U.S. Embassy in screening Cambodian officials several years ago, said he was concerned by the failure to weed out questionable applicants. “When you see that a person who has violated human rights gets training, it seems he is given encouragement, and this is not a good thing,” he said. “It fuels him to violate human rights even more.”

At the time of the 2007 evictions in the seaside province, U.S. officials had already suspected that human rights vetting of local security forces was a problem. In January of that year, a team from the U.S. State Department Inspector General’s office visited the Phnom Penh Embassy and later reported the screening process had been “cursory and uneven.”

In practice, the Congressional vetting requirement, known as the Leahy law after its principal sponsor Vermont Senator Patrick Leahy, has meant that background checks are required for all trainees. But the State Department report found the embassy kept no records of its local checks and used out-of-date guidance. Other agencies involved in the training didn’t understand the process and failed to submit the names of trainees for investigation, it said.

The State Department said it could not comment on individual cases. “We take our obligations under the Leahy vetting process seriously,” Julia Straker, a spokeswoman, wrote in an email. “Consistent with U.S. law and policy, the Department of State vets its assistance to foreign security forces, as well as certain Department of Defense security assistance programs.”

Cambodian officials would not discuss the human rights records of those trained by the United States. Attempts to speak with spokespersons for the Interior and Defense ministries were unsuccessful. Senior military officers referred questions to General Meas Sophea, the infantry commander. Through an assistant, Gen. Sophea declined to comment.

Lork Kheng, a member of the governing Cambodian People’s Party and of a National Assembly panel on human rights, defended her country’s security forces. “Generally speaking, I believe all commanders carry out their duties in compliance with the law,” she said in an interview.

Phay Siphan, spokesman for the Council of Ministers, a civilian policymaking body, said that in Cambodia anyone complaining of abuse by the military could seek legal redress.

“We are a member of the United Nations and have our own rule of law,” he said. “We do have lawyers; we do have a court system. Whoever is abused can file to the court. That’s the way the rule of law is.”