To the hundreds of students who lined up outside the funeral home that April evening in 2010, Brian Betts had been a beloved Washington, DC, middle school principal. A second father. An inspiration.

“R.I.P. Mr Betts,” said their shirts and hoodies.

“Mr Betts, We Love You,” read their signs.

But to O'Neil McGean, who stood in the Pierce Funeral Home parking lot in Manassas, Virginia, gripping a friend's hand and fighting back tears, Brian had been so much more.

He had been the love of O'Neil's life.

They had met at a stoplight, O'Neil's personality so boisterous it took him only a few seconds to make a lasting impression. Soon they bought a house together in the District, fixing it up in the evenings. They were inseparable for almost a decade. And even after their breakup, after O'Neil moved to Mexico and Brian moved to Maryland, they remained good friends.

Then came the gunshots late one night inside Brian's bedroom in Silver Spring, Maryland, and the phone ringing 2,000 miles away in Mazatlan.

A week later, O'Neil stood in front of his ex's casket wondering what had gone wrong.

“Why did this happen, Brian?” O'Neil asked aloud.

The answer came two weeks later when police arrested four men, one of whom had arranged to meet Brian via a telephone chat line only to rob him, shoot him and leave him to die.

The crime chastened O'Neil. He was already careful about living in Mexico. Now he grew wary of online dating.

But by 25 October 2016, that caution had waned. After agreeing to meet someone through a dating app, O'Neil disappeared - as did $16,000 from his bank accounts.

The question this time was less why than how.

How could O'Neil fall prey to the same trap that had claimed Brian six years prior?

How could the 53-year-old not see it coming?

The first messages weren't alarming.

“Hola amigo, you there,” Jorge Guillen Gonzalez wrote on Facebook messenger on 26 October 2016.

“Si, yo estoy aqui,” replied Donnie McGean, O'Neil's oldest brother. “All is good, and you?”

“Not as good [as] I want.”

They had met six months earlier when Donnie and his wife visited O'Neil in Mazatlan, a city known as the Pearl of the Pacific.

O'Neil had moved there in 2006 after visiting a few times. The same charisma that had made him the centre of attention as a kid in Chevy Chase, Maryland - leading his little brother Chris and their friends through Rock Creek Park, refereeing fights after school at Blessed Sacrament, captaining dodgeball games - made him popular in the gay-friendly resort town.

It was in Mazatlan that O'Neil met Jorge, a handsome young Mexican with dark hair, green eyes and a tattoo across his tightly muscled chest reading “Warrior of God.” They had dated for a short time before opening a cafe together in 2014.

Now Jorge said he was worried.

O'Neil had gone on a date the night before with someone he'd met on a gay dating app, Jorge said, and O'Neil wasn't home yet, nor was he answering his phone. His two cherished dogs - Brandy and Guinness, named after O'Neil's favourite drinks - hadn't been fed.

Drug violence in the surrounding state of Sinaloa had crept into Mazatlan. So when Jorge said he was receiving strange Spanglish texts from O'Neil's phone, Donnie told him to call the police.

“[I] really miss O'Neil. He is my life. He knows how much I love him. Hope he is OK, wherever he is,” Jorge wrote in broken English.

“My heart is broken,” he said later. “I just wanna die.”

“Hang in there. I will be there tomorrow,” Donnie wrote as he prepared to board a flight from his home in Maui to Mazatlan. “Our family is very grateful to have you as a friend of O'Neil. Without you we would be nowhere right now.”

Twenty hours later, Donnie, an energetic 62-year-old who founded a trio of natural food groceries, stepped off a plane and headed to meet Jorge at the Hotel Punta Pacifico, a remote resort north of the city. It was here, Jorge said, that O'Neil had gone to meet his date the night he disappeared.

But hotel employees denied seeing O'Neil, and drone footage of the surrounding countryside showed no trace of him or his car.

The sun dipped over the ocean as Jorge drove them south to Mazatlan. They were eating seafood at a restaurant on the malecón when the Mexican's phone suddenly began to buzz.

The messages were from O'Neil's phone - but not from O'Neil.

“Pay great attention because I will not say it again,” the kidnappers said in Spanish. “If it occurs to you to do something rash, you will not hear from me or your little sponsor again.”

The kidnappers had already withdrawn about $16,000 from O'Neil's bank accounts. Now they demanded $26,000 more, but gave confusing directions, first instructing Jorge to pay in the morning, then ordering him to deposit a fifth of the money immediately - without providing a bank account.

“I won't do anything until I see a photo that O'Neil is OK,” Jorge wrote back.

“First hand,” came the cryptic, chilling answer. “Want the other? You don't give the orders here.”

As the texts became more threatening, Jorge grew visibly distraught, sobbing and retching, recalled Donnie, who was busy dialing FBI and Drug Enforcement Administration (DEA) officials - contacts of a relative who'd retired from the DEA - to ask them to try to trace his brother's phone.

The next morning, someone spotted O'Neil's car, parked downtown and filled with trash and beer bottles. As Donnie and Jorge watched state police dust the car for fingerprints, an officer pulled the American aside to say he had a bad feeling about Jorge.

Donnie shrugged it off, as he did the other things people said about Jorge: that O'Neil had recently fired him from the cafe; that he'd been banned from O'Neil's house for throwing wild parties while the American was away.

Jorge went everywhere with Donnie, translating for him by day and sleeping in the same house at night. He even suggested suspects to police, organising a stakeout at a property where he said O'Neil might be held, Donnie recalled.

O'Neil had been a popular figure in Mazatlan, donating money to local causes and hosting events at his cafe, so his disappearance was major local news. On 31 October, Donnie's fourth day in town, he and Jorge went to meet the mayor. Carlos Felton told Donnie he'd spoken that morning to the governor, who had made it clear he wanted the case quickly solved.

“A lot of these guys were very afraid that this would affect their tourism, would affect the cruise ships,” Donnie later recalled.

The same day, Donnie met with the prosecutor handling O'Neil's disappearance.

“What took you so long to come in here?” Agripino Flores Sanchez asked. “We told Jorge a family member has to sign off on the investigation.”

The next day, when Donnie returned to talk to Flores, the prosecutor barred Jorge from entering the room. He then showed Donnie a diagram of communications between the suspected kidnappers. Jorge's name appeared, Donnie recalled.

Donnie again dismissed the idea. Jorge must have been trying to reach the kidnappers to negotiate O'Neil's release, he thought.

The next day Mazatlan was packed with people celebrating the Day of the Dead. To take his mind off his brother's disappearance, Donnie walked among the thousands of partygoers with their faces painted like skulls before ducking into a restaurant to call a kidnapping expert.

“If you're continuing to be hopeful, don't,” the expert said after Donnie told him the kidnappers had gone quiet. “I'll tell you right now that your brother is dead.”

Donnie's phone rang just hours after he'd left Mazatlan.

Police had found O'Neil's body, his youngest brother, Chris, told him, and they had arrested Jorge.

Six years after Brian's murder, O'Neil had fallen prey to a similar trap - one allegedly orchestrated by his best friend.

O'Neil had been lured not to the Punta Pacifico but to another hotel, where he had been beaten so badly that his lungs were punctured, investigators told Donnie. His brother's body was then wrapped in a hotel curtain, stuffed inside a large bag, ferried across town in a taxi and buried in a yard under freshly poured concrete.

The FBI agent had warned him not to look at O'Neil's face, so Donnie identified his little brother by the Irish family crest tattooed on his shoulder.

Mexican law does not allow Mexican media to fully identify suspects until they have been convicted. But multiple people close to the situation, including investigators and an attorney for Jorge, confirmed his arrest and those of two others: Luis David Soto and Carlos Ramon Anguiano. A fourth suspect, Joel Carrillo Anguiano - a relative of Carlos - has also been charged but remains at large.

State and local authorities did not respond to repeated requests for comment.

In an interview with The Washington Post, Jorge's attorney, Hector Soto, said his client had been made into a “scapegoat” by officials eager to close a high-profile and politically sensitive case.

Jorge had sounded the alarm over O'Neil's disappearance and pressured police to investigate, Soto argued. A confession by Carlos Anguiano implicating Jorge was unreliable, he said.

“Carlos says he was tortured into giving that statement,” Soto said.

That accusation cuts deep in a country that has struggled to modernise its outdated, underfunded and, at times, corrupt criminal justice system. Despite a decade-long effort to bolster the rule of law by improving policing and introducing American-style oral court proceedings, more than 93 percent of homicides go unsolved, according to the Citizens Council for Public Security and Criminal Justice, a Mexican think tank.

On the rare occasion that a homicide is closed, it is often tainted by accusations of torture, as in the case of two Australian surfers killed in Sinaloa a year before O'Neil.

In a jailhouse letter sent to The Post by his brother, Jorge claimed he is innocent.

“I'm locked up because of the whims of prosecutors and the disabilities of judges,” he wrote. “I'm locked up because the state government wants to get along with the American community.”

Donnie McGean believes Jorge is guilty. Rather than signs of innocence, he sees Jorge's retching and crying as evidence he knew the robbery had gone too far, and O'Neil was dead. But even he isn't certain.

“In Mexico, you cannot trust anybody,” Donnie said, “including the police.”

A year after his brother's death, Donnie and his relatives worry that the case will fall apart. The governor who had prioritised O'Neil's case left office last year amid accusations of corruption.

“I feel that the case is being put on the back burner,” Donnie wrote to Sinaloa's new governor, Quirino Ordaz Coppel, in May. “A kidnapper, robber and murderer of an American living in Mexico is still walking the streets.”

Donnie never received a response. Later he learned Ordaz, who did not respond to requests for comment, owns the Pacific Palace hotel, where O'Neil was killed.

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A month after O'Neil's body was found, expats drank tequila and sang “Danny Boy” at a memorial in Mazatlan. On the same day in Washington, mourners packed Blessed Sacrament for a memorial just as emotional as the one held for Brian.

Earlier this year, when Chris and Donnie went through their brother's belongings, they found dozens of children's books Brian had given O'Neil, each with a love note written inside.

And in his dressing room in Mazatlan, framed behind glass, they found a collage of photographs of O'Neil and Brian - both of them now gone.