It happened in an otherwise peaceful park, along a beautiful stream, flowing down to a famous river -- the Potomac.
In fact, the stream-side park lies just across the road from a neighborhood that has been home to an attorney general, a future Republican presidential nominee, a Democratic White House chief of staff and a liberal justice of the Supreme Court.
The boy's mother, according to reports by The Washington Post and Washington's local NBC affiliate, said he left their apartment -- "in his pajamas," as the Post noted -- to drop some trash in a dumpster. But he did not come back.
That was Sept. 26, 2016.
About six weeks later, with 14-year-old Sergio Triminio still missing, police arrested an MS-13 gang member, according to an affidavit that an FBI officer submitted to the U.S. District Court for the Eastern District of Virginia.
He went by the nickname "Lil Sicario" -- and he had his cellphone on him. It contained a hideous video.
"In the suspected murder video, the victim can be seen lying face down on the ground while multiple subjects are striking the victim with weapons, including a kitchen knife and a machete," says the FBI affidavit.
"In the video, the victim is wearing plaid boxer shorts and multi-colored pajama pants," it says. "A close-up of the victim shows that a piece of the victim's scalp has been sliced away by a sharp object. During the attack, the assailants are heard speaking in Spanish. The video lasts approximately 52 seconds, ending while the attack is still underway. After reviewing the video, Fairfax County detectives suspected that the victim in the video was either Triminio or another individual recently killed in a suspected MS-13 murder."
About three and a half months after that, on the last day of February 2017, according to the FBI affidavit, an "individual known by law enforcement to have ties to the MS-13 gang ... advised a Fairfax County detective that he knew where two bodies were buried."
This "confidential source" -- or CS-1, as the affidavit calls him -- brought the police to the Holmes Run Stream Valley Park.
This park, a densely wooded ravine, sits directly across Columbia Pike from the Lake Barcroft neighborhood -- former home to LBJ's Attorney General Ramsey Clark, RNC Chairman Bob Dole, Clinton Chief of Staff John Podesta and Supreme Court Justice Thurgood Marshall.
The park is figuratively -- if not literally -- in the shadow of the Capitol. If you drive down Columbia Pike to the 14th Street Bridge, it is about 9 miles to the Supreme Court and a little less than that to the office buildings of the U.S. House of Representatives.
Along the way you will pass the Jefferson Memorial, where an inscription says: "all men are ... endowed by their Creator with certain inalienable rights."
On March 1, 2017, the FBI joined local police in searching for the two bodies CS-1 said were in the park.
They found them. "Among one set of remains, officers found plaid boxer shorts, multi-colored pajamas, and a kitchen knife," says the affidavit. "These items of clothing were similar, if not identical, to those worn by the victim in the aforementioned video," it says. "The recovered kitchen knife also appeared to be the same one law enforcement observed in the aforementioned video."
A pathologist confirmed that this was Sergio and that he had been murdered.
Then the confidential source unloaded.
"CS-1 told investigators that he was present for Triminio's murder," says the affidavit.
"According to CS-1, several MS-13 members and associates stabbed Triminio with knives and also struck him with a machete and pick axe," it says.
"CS-1 told investigators that Triminio's murder was recorded on video and sent to 'Lil Sicario,' who approved the murder," says the affidavit.
Why does the FBI think they did it?
"Investigators have obtained evidence indicating that Triminio was an associate of the international street gang, La Mara Salvatrucha, also known as MS-13, and that Triminio was murdered by other MS-13 members and associates because they believed he was cooperating with law enforcement," says the affidavit.
An indictment was unsealed last week in the U.S. District Court. The U.S. attorney's office said in a press release that it alleges that 10 men "conspired together and with others to lure a 14-year-old male to the same park in Fairfax County where he was attacked and killed because he was thought to be cooperating with law enforcement."
The U.S. attorney said that El Salvador was the "country of origin" for all 10 men. Six of them, according to the U.S. attorney's press release, are now only 20 years old -- meaning they were 18 when the alleged murder occurred.
Sergio Triminio, whom The Washington Post reported "was born in Honduras," was denied his God-given right to life before he had lived even that long.