On September 23, 2013, 55-year-old Katherine Grgich set off from her guesthouse on the tourist island of Koh Rong to trek through its jungle interior in search of a secluded beach. It should have been a leisurely hike for the Southern Californian masseuse—a martial arts enthusiast and surfer whose fitness belied her age. But she did not return.

When she failed to check out of Sunset Bungalows, a group of locals, tourists and business owners formed a search party and the following morning, on September 29, her decomposing body was found lying off a narrow path through the forest.

Sihanoukville police investigating on the island initially reported the death as an accident, a conclusion carried by both local and international media. But according to police reports, they quickly revised their assessment and determined that Katherine Grgich had been the victim of an aggravated robbery that had escalated to murder.

Though it was not made public until several weeks later, police identified a suspect within a few days—Yan Yoeun, then 32, an odd-job man living on the island with his wife and two children who had a history of violence and drug addiction and a reputation for robbing tourists on the underdeveloped island.

According to the suspect’s wife and other witnesses, he had been working alongside his brother-in-law Theoun cutting logs in the hills beyond Koh Touch village around the time of the murder, close to where the body was found.

Shortly after Katherine Grgich disappeared, the suspect had given his wife a camera to sell—the same camera that the American had been seen carrying when she left her guesthouse. On the day she was found, Mr. Yoeun abruptly fled the island by boat, leaving his wife and children behind. He was last seen boarding a bus to Koh Kong province with his onward destination presumed to be Thailand.

The Preah Sihanouk Provincial Court issued a warrant for Mr. Yoeun’s arrest on October 18, having ruled out the involvement of his brother-in-law. Provincial police and Interior Ministry officials are still convinced that Mr. Yoeun is the killer, but admit that 14 months on they have made no progress in finding him.

“We know the suspect killed her to get the camera and that he tried to sell it, but when no one bought, he left it with his wife before escaping,” Lieutenant General Mok Chito, director of the Interior Ministry’s central judicial department, said last month. “But the suspect remains at large, as we do not know his exact whereabouts.”

Kol Phally, the deputy provincial police chief in charge of the investigation, said his officers acted quickly after learning the suspect had escaped the island, immediately alerting police at the Cham Yeam border checkpoint. But by then, Mr. Phally said, he had likely already crossed into Thailand.

“Yan Yoeun is 100 percent the suspect. He has gone across the border and may now be a migrant worker, but we do not know the exact place he is staying, though we are continuing to search for him,” he said.

Mr. Phally did not elaborate on the methods Cambodian police were using to carry out such a search, but he conceded that neither Thai nor international police had been informed that the suspect may have entered Thailand.

“We have not cooperated with the Thai police. When we know which province [in Thailand] he lives in, we will seek their cooperation,” he said, adding that the U.S. Embassy was being kept abreast of developments in the investigation.

The spokesman for the Thai police, Lieutenant General Prawut Thawornsiri, said last month that he was surprised that Thai authorities had not been informed of the case, either immediately after Sihanoukville authorities believed Mr. Yoeun had gone to Thailand or in the intervening period.

“I have never heard of this case and never heard the name of this suspect mentioned,” he said.

Lt. Gen. Prawut added that in similar circumstances, Thai police would have asked for Cambodian assistance and also immediately made Interpol aware of the case.

“This is good and proper protocol to follow. We would contact the Cambodian Embassy and let them know. And when it is a murder case, Interpol would become involved,” he said.

Lim Sokha Raksmey, acting director of Cambodia’s Interpol office, said it was the responsibility of provincial police to inform the Interpol office—and in the case of Mr. Yoeun they had not done so.

“I have no idea about this murder suspect. If a request comes through to us, we put it through the Interpol system, which flags a red notice and activates the case,” he said, adding that he did not know why Interpol had not been notified.

Cambodian authorities are not always so shy about requesting Interpol’s assistance.

In early December, they asked for the agency’s help to find Major General Thong Sarath—a Defense Ministry official who fled Cambodia and is still on the run after being accused of the murder of Ung Meng Chue, a wealthy businessman who was gunned down outside a fruit store in Phnom Penh in November.

“We are working with our Interpol office,” Chhay Sinarith, director of the Interior Ministry’s internal security department, said of the search for Maj. Gen. Sarath. “When we ask them to help, they have to look into a case.”

Even without engaging Interpol, informal cross-border cooperation between Thai and Cambodian security forces can sometimes yield quick results.

In October, Thai police informed authorities in Koh Kong that a 35-year-old Dutchman suspected of beating his Thai wife to death a few days earlier had likely crossed into Cambodia via the same Cham Yeam border checkpoint that Mr. Yoeun is believed to have crossed.

Within 48 hours, he was detained by Cambodian military police and transferred into Thai custody. Thai police had sent photographs of the suspect and a copy of his passport to their counterparts in Koh Kong via the mobile phone application WhatsApp, making it easy to locate and identify him.

A week after reporters brought the case of Mr. Yoeun to his attention in December, Interpol’s Mr. Sokha Raksmey said that he felt compelled to reverse the usual procedure and had personally asked Sihanoukville authorities to send him the case file on Katherine Grgich’s murder.

“I requested that the [Preah Sihanouk provincial] court send me the case and now it will be put in the system. We have the suspect’s name and his picture and it will be logged regionally, not just in Thailand,” he said.

Grief Becomes Frustration

Katherine Grgich’s sister, Jennifer Grgich-Harden, who spoke on behalf of her grieving family back in Santa Barbara, California, said she was now hopeful that the belated involvement of Interpol would rekindle the investigation into her sister’s death.

But the family’s growing frustration with the investigation’s lack of progress has been exacerbated by the failure of police to pursue the most obvious channels to search for the suspect, suggesting they had little genuine interest in finding him, she said. Furthermore, the U.S. Embassy’s repeated assurances to her that they were raising the case at the highest levels of Cambodia’s government began to sound empty.

“It’s baffling,” she said of the fact that the Embassy had not—at the very least—pushed for Interpol to be alerted. “What role does our U.S. Embassy hold if they do nothing at all?!?!”

Responding to questions regarding the investigation, the embassy said that notifying Interpol was a matter for Cambodian police because although the U.S. Embassy houses law enforcement “entities,” they are not legally authorized to investigate the murder of a U.S. citizen occurring outside U.S. jurisdiction.

“The investigation is the responsibility of the Cambodian authorities,” said Courtney Woods, assistant public affairs officer at the embassy. “It is incumbent upon the Cambodian authorities to notify Interpol, as the suspect was identified through their investigation and would be tried within their legal system.”

The embassy was, however, doing everything it could to advance the case and was keeping Katherine Grgich’s family in the loop, he said.

“In our role as a diplomatic mission, the Embassy has actively raised the case at senior levels of the Cambodian government. Federal law enforcement agents at the U.S. Embassy continue to liaise regularly with Cambodian National Police leadership to seek updates on the investigation…and we communicate those updates to the family directly.”

Approximately 60 to 70 U.S. citizens die of various causes in Cambodia each year, according to Mr. Woods. When a next of kin is not present, the embassy will send a representative to identify the citizen’s remains and notify a family member of the death; all consular officers receive training on how to handle death cases as sensitively as possible and help them make the necessary arrangements, he said.

But over the past year, the U.S. Embassy in Cambodia has exacerbated the Grgich family’s pain instead of helping to ease it, said Ms. Grgich-Harden.

“It appears to us that nothing is being done to find the suspect, because even though we have asked via our embassy numerous times, we have not been given any specifics,” she said.

“I am my sister Katherine’s voice representing my grieving family but I want people to know that my parents—who are both in their eighties and too devastated to deal with this—are not satisfied with how my sister’s death was handled and also the fact that we all feel helpless being so far away.”

Adding to the family’s exasperation were reports from tourists who had been present on the island when her sister’s body was found, and who began getting in touch with Ms. Grgich-Harden to express their concern about a special agent sent to Koh Rong by the U.S. Embassy.

The agent was with the local police officers when they wrongly noted the cause of death as indeterminable and the result of an accident—and did not contest the finding despite the protests of witnesses at the scene, according to those present. Two days later, on October 1, Katherine Grgich’s body was cremated on the spot where it was found.

“It was a very shady incident at [the] time,” Brent Clarke, an American tourist who was involved in the search, said via email. Mr. Clarke did not see the body personally, but said the atmosphere on the island quickly darkened when foul play was ruled out.

“A large number of us left in the days following it, because rumor was the death was being covered up and made to look like an accident—people I’d spoken with said there was clearly foul play involved,” he said.

A week later, the U.S. special agent personally contacted Ms. Grgich-Harden. He explained that the condition of Katherine’s body, coupled with a lack of technical expertise, made it impossible to ascertain the cause of death and warned that the family might never learn the truth about how she died.

“Cambodia is an extremely poor country, and despite the [police’s] best efforts, we may never know exactly how your sister died,” he said in an email to her on October 8, adding that he was not assisting with the investigation as he had no legal authority to do so.

Ms. Grgich-Harden said it was initially a relief to talk with the agent. Her family was still in shock and felt adrift, knowing little about Cambodia and bereft of details about what had happened to her sister. The agent seemed to offer a much needed first-hand account of events on the island following her death, and it consoled them to know he had helped take care of her remains.

But her opinion soon changed when she saw photographs of her sister’s body online in Cambodian media that conflicted with information he had given her. Moreover, an official at the Cambodian Embassy in Washington had already told her that foul play was considered likely.

“Witnesses who were on the island expressed to me that the way in which my sister’s death was handled was wrong,” she said. “They listened to the communication between the “Special Agent” and the Cambodian police [and] at least one person left the island because he was so disgusted at how my sister’s death was ‘managed.’”

One of those witnesses was then-22-year-old Katherine Powell, a British backpacker who was among the first to see Katherine Grgich’s body. She said it bore clear signs of a violent attack and possible sexual assault, as her clothing had been partially removed.

“I was shocked and horrified as soon as I saw her,” she said via email last week, adding that she lodged a complaint with the U.S. Embassy to officially contest the noted cause of death corroborated by the U.S. agent.

“It looked like she had been flung from the trail by her hands and feet,” Ms. Powell said. “There is no doubt about it, Katherine Ann Grgich was murdered and most probably raped as well. This was no accident.”

An Australian business owner on the island named David Mitchell, who helped lead the search and stayed with the remains as police examined the crime scene, said the Sihanoukville police who first appeared on the scene were eager for the U.S. Embassy official to arrive so that he would take control of the situation.

“The Cambodians were waiting for the big embassy guy to get here to take over the investigation, but when the embassy guy got here he seemed to be a little out of his depth,” Mr. Mitchell said.

“He said it wasn’t his job to investigate and told the Cambodians to just write up the report. We all told him the cause of death should be ruled suspicious but he didn’t push [the police].”

Soon after the first report was written up and the agent left the island, Sihanoukville police realized that the cause of death could not have been accidental and began a murder investigation, working quickly thereafter to identify a suspect and bring the case to court.

Ms. Grgich-Harden had already learned that police had issued an arrest warrant via Cambodian media reports by the time the U.S. Embassy in Cambodia updated her. And despite the progress made by police at that stage, a U.S. consular officer continued to blame a lack of Cambodian expertise for the dearth of updates.

In Ms. Grgich-Harden’s opinion, the embassy was either not interested enough in the case to ask for updates, or was deliberately withholding information. In the coming months, her frustration deepened.

“I was so disappointed and let down by our government, not only that I had been told mistruths, but I hadn’t heard from our Embassy since December [2013] and in April [2014] I finally had to contact them,” she said.

“I felt like my sister had been tossed away like a piece of trash, dead, forgotten and without a voice.”

This October—a little over a year after the arrest warrant for the suspect was issued—the embassy repeated in an email that they were trying their best to move the case forward and told her that U.S. Ambassador William Todd had personally raised the issue at the highest levels of Cambodia’s government.

“Your sister’s murder has not been forgotten and we will not allow it to be forgotten,” a consular officer wrote in an email. “I must be frank though and also state that, given the extremely limited abilities of local authorities to investigate crimes like your sister’s murder, we may never get the specific information you are seeking.”

A similar email followed in mid-December, but with Cambodian authorities certain that the perpetrator had been identified a year earlier, and with a warrant issued, the only issue left to be raised was why Interpol and Thai authorities had not been informed.

“It has become more and more evident that they want to sweep this under the rug and have it go away,” Ms. Grgich-Harden said.

Unrealistic Expectations

The relationship that develops between families of loved ones murdered abroad and their respective embassies is often strained, especially in a country such as Cambodia, where the families can feel cut off by distance, language and cultural barriers.

In recent years, several drawn-out cases in the country have provoked paranoia among frustrated relatives for whom the perceived standoffishness of consular staff amid the lack of progress in the investigations translates as evidence of their complicity in a cover-up, or unwillingness to extend their influence.

In many instances, the warnings repeated to Ms. Grgich-Harden that Cambodian police lack the capacity to carry out a credible investigation rings painfully true.

In early 2012, the bodies of Frenchman Laurent Vallier and his four young children, who had disappeared three months earlier, were discovered inside a car submerged in a pond near the family’s home in Kompong Speu province.

Police quickly determined that the father drove his children into the pond in a murder-suicide, which appeared to conflict with information reported by local media about the crime scene. Laurent Vallier’s family, who suspected he and his children had been victims of an ongoing dispute with his late Cambodian wife’s relatives over land, complained to the French courts, which dispatched a team of forensic specialists the following year.

Within days of their arrival, they ruled out suicide and the results of an autopsy released in August 2014 substantiated the cause of death as murder. The team also examined the site where the body of another French national, 25-year-old Ophelie Begnis, washed up on a Kampot City riverbank in February 2013.

Murder was indisputable. In that case, police also quickly fingered a perpetrator—Belgian national Olivier van den Bogaert—who was placed near the scene by a sole witness. But the charges against him were dropped several months later when DNA tests carried out by a forensics team back in France proved inconclusive and no other evidence linked him to her death.

With no new leads being investigated, her grieving family traveled to Phnom Penh in November last year, breaking their media silence to criticize the French Embassy for a lack of communication and impetus, which the family’s lawyer deemed “unacceptable and unworthy of a state like France.”

Lacking the resources to hire a lawyer to maintain that kind of pressure, the family of William Glenn, a 42-year-old American teacher found dumped on the outskirts of Phnom Penh in July, resigned themselves to never knowing who killed him or why.

His wife, a Thai woman living in Bangkok from whom he had recently separated, grew increasingly frustrated and suspicious at the U.S. Embassy’s perceived aloofness and its failure to keep her in the loop, despite the fact that she provided them with their marriage certificate to prove she was his legal next of kin.

“How can they do this to me?” she said in the months after his body was found. “I am his wife, but they do not tell me anything or say strange stories that make me not trust them.”

She added that her husband had gone to the embassy in Phnom Penh on the morning he was last seen alive to request extra pages in his passport, as he told her his life was in danger and he needed to leave the country. But she said the embassy confiscated his passport and her appointed consular officer declined to answer any questions relating to their decision.

And even when media coverage does successfully throw a spotlight on a questionable conclusion by Cambodian police, or casts doubt on the rigorousness of their efforts to investigate suspicious deaths, it is no guarantee of a satisfactory outcome.

Friends of the late Canadian journalist Dave Walker, whose decomposed body was found in March on the grounds of the Angkor Archeological Park in Siem Reap City three months after he disappeared, continue to insist that the evidence points to murder, and say Cambodian authorities deliberately forestalled a proper investigation.

But for his family back in Canada, their need for closure, coupled with the unlikelihood that his death would ever be explained, persuaded them in September—after paying for their own autopsy to be carried out—to accept that the case was closed.

“Canadian authorities never treated the case as criminal so my family and [private] investigators worked…along with a lawyer living in Cambodia who was helping me all pushed for the case to be criminally investigated,” said Tammy Madon, a cousin of Dave Walker who lives in Edmonton, Canada.

“My family and I do not wish to draw this out any further. I don’t believe we will ever find out what happened due to many reasons, which include the unwillingness of the authorities and the reluctance of the Canadian Authorities to step in and investigate further.”

Back in Santa Barbara, Katherine Grgich’s family is still clinging to the hope that justice in their case will be served and pray that they will have meaningful closure when her killer is brought to trial.

Shortly after Christmas, Ms. Grgich-Harden’s belief in the U.S. Embassy was rekindled when a senior consular official spoke with her by telephone and expressed his dismay at the testimony of the travelers on the island regarding the blatant evidence facing the U.S. agent as he signed off on an accidental cause of death.

He promised her that Ambassador Todd would press every angle on the case during his meeting in mid-January with Interior Minister Sar Kheng. But on January 30th, Ms. Grgich-Harden was once again disheartened after the embassy responded to an email informing her that as of then, no meeting had been scheduled.

“My sister’s voice still resonates inside of me: ‘Isn’t anyone going to do anything?!? I was killed, thrown away and forgotten,’” she said earlier this week.

“It is time for me to stand up and have my sister’s voice heard and I am not going to let the Cambodian government or the U.S. Embassy sweep her murder under the carpet like it didn’t happen and that she didn’t matter.”

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