Sergio Triminio, 14, agreed to meet the brother in September 2016 and told him a chilling story: Mendez, 17, of Falls Church, Virginia had been abducted by MS-13 and killed because they thought he was a spy for their archrival, the 18th Street gang.
The next day Sergio disappeared, too, after leaving his family's Alexandria, Virginia, apartment in his pajamas to take out the trash, the court documents say. He was also targeted by MS-13, possibly for speaking with Mendez's brother.
The bodies of the two teens were discovered in unmarked graves in Fairfax County's Holmes Run Park in February, two more young victims in a wave of violence by the brutal street gang, which has gripped the Washington region and sparked national concern.
Sergio's mother, Karla Triminio, said Friday that the grief has been unbearable for the family. She said her 3-year-old daughter keeps asking where her older brother is.
"I tell her that he's in heaven," Karla Triminio said. "But then she goes around looking for heaven, looking for him."
The search warrants state Mendez received several calls on Aug. 28, 2016, from an unknown person. Mendez ignored the first two but answered the third. A family member told detectives she overheard Mendez saying, "Wait for me, I'll be right there."
Mendez immediately left his family's apartment and was never seen again. On Sept. 1, his family officially reported him missing. In mid-September, family members reached out to police, saying they thought Mendez's disappearance might be gang-related, according to the search warrants filed in Fairfax Circuit Court.
Then on Sept. 25, 2016, Mendez's brother met with Sergio at an Alexandria hotel, seeking information. The search warrant states that Sergio was an associate of MS-13, which is why he knew details of Mendez's killing.
Mendez's family, which is from Guatemala, could not be reached for comment.
Karla Triminio said she sent Sergio to take out the trash around 8 p.m. on Sept. 26, 2016. When he didn't return after about 15 or 20 minutes, his mother said she began texting and calling him but got no response.
At the time, Sergio was on probation and was wearing a court-ordered ankle bracelet, according to the search warrant. But after Sept. 26, 2016, the ankle bracelet's signal could not be detected, according to the search warrant.
On Oct. 4, Sergio was officially reported missing.
Karla Triminio said she thought Sergio had been approached by gang members at school before he went missing, leading to him having disciplinary problems. He began disappearing from home, so she went to police and juvenile court for help. She said Sergio was born in Honduras, before the family came to the United States.
Triminio said she suspected MS-13 might be involved in her son's disappearance because a female gang member approached her after Sergio vanished and offered to help find him. The mother said she concluded the gang member was actually trying to see what information she had relayed to police.
In mid-October of 2016, according to the court documents, police interviewed someone who said Sergio had a photo of Mendez and said he had been murdered around Aug. 28, 2016. Sergio did not tell the person what prompted the killing.
In December 2016, police developed information that Mendez and Sergio were killed and buried in Holmes Run Park in the Lincolnia area of the county. Police said the gravesites were found about 300 yards into the park on Feb. 28, 2017. Fairfax County police announced the discovery of the bodies on March 3.
The location of the graves has an eerie resonance because MS-13 killed and buried two wayward gang members in Holmes Run Park in 2013. Those murders became part of a high-profile federal prosecution that cracked down on MS-13 in Northern Virginia.
In recent years, MS-13 has reconstituted itself both in Virginia and Maryland. The gang, which has some 900 to 1,100 members locally, is the region's biggest and most violent gang.
MS-13 has been involved in grisly killings in recent years that have often targeted teens. Earlier this year, police found the body of 15-year-old Damaris Reyes Rivas near a Springfield, Virginia park.
Police say she was attacked with a knife and a large stick in a brutal killing that MS-13 members recorded on cellphones. The video was recently played in court during the trial of one her killers. The girl's shaken mother cried.
A handful of people have been convicted of charges in the case, while others are awaiting trial.
In Maryland earlier this month, Montgomery County prosecutors detailed a particularly violent MS-13 killing during which an unidentified man was stabbed more than 100 times, decapitated and had his heart ripped out. Police believe as many as 10 gang members were involved in that slaying earlier this year.
Karla Triminio said the past year has been one of terrible anxiety.
"The pain never ends," she said.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)