Christian Lemus Cerna, an MS-13 gang member, allegedly set out into a wooded park in Fairfax County in May 2014 to find a pair of graves. With him was a local MS-13 leader, known as Junior, who wanted to spook some new recruits by showing them where the bodies of two wayward members were buried.

The previous October, Cerna’s clique had allegedly lured one member into the park, stabbed him to death, hacked his body up and buried him. Several months later, they did the same to another member who broke gang rules, according to prosecutors.

Junior asked Cerna, then an 18-year-old in high school, to show him the grave sites. Cerna obliged. The men walked in the park for half an hour before they came to the right spot. Cerna told Junior that he’d have to hold his nose. “That stuff is like a skunk,” he said.

What Cerna didn’t realize was that Junior was a paid confidential informant for the FBI. He had infiltrated Cerna’s clique months prior and wore a video wire during the trip into the park, recording the entire meeting.

Junior is a star witness for the government in a criminal case underway accusing MS-13 members of murder, attempted murder and conspiracy. The 33-year-old has worked for the FBI for the past decade, earning more than $40,000 and providing authorities with a critical view into the inner workings of one of the Washington area’s most violent street gangs. The Washington Post is not identifying Junior by his real name at the request of prosecutors, who said he could be in danger.

1 of 9 Full Screen Autoplay Close Skip Ad × Faces of MS-13 View Photos The Mara Salvatrucha gang, the transnational gang known more colloquially as MS-13, is involved in criminal activities in most parts of Latin America, as well as the United States. The gang originated in Los Angeles and spread from there, and now has a large presence in Fairfax and Prince George’s counties. Caption The Mara Salvatrucha gang is involved in criminal activities in most parts of Latin America, as well as the United States. Sept. 7, 2006 A member of the Mara Salvatrucha gang, known more colloquially as MS-13, is presented to the press in San Salvador after his arrest. Some 130 gang members suspected of committing homicides were arrested during a police operation in different departments of the country. Yuri Cortez/AFP/Getty Images Buy Photo Wait 1 second to continue.

In more than 15 hours of testimony last week, Junior described how he secretly recorded hundreds of phone conversations and in-person meetings with the defendants, during which prosecutors say gang members admitted their roles in the alleged crimes.

Thirteen alleged gang members have been charged in connection with the two killings in the park as well as a shooting in Alexandria and an attempted slaying in Woodbridge, Va.

A trial for half of the defendants began in late March in the U.S. District Court for the Eastern District of Virginia. Six defendants have accepted plea agreements, and some of them are cooperating with prosecutors. One defendant will be tried separately.

[Alleged MS-13 members accused in three slayings in Northern Virginia]

Junior’s cooperation enabled prosecutors to charge a large group of defendants at the same time. But it also sheds light on the complications of building a case that relies heavily on testimony of a known gang member and immigrant trying to gain legal status.

In court last week, Junior was soft-spoken and clean-cut. He said he came to the United States from El Salvador in 2000, crossing the border illegally and settling in Northern Virginia. After dropping out of high school as a senior, he was recruited into an MS-13 clique called the Silvas Locos Salvatrucha.

[Video: El Salvador's rival gang leaders discuss recent cease-fire]

At some point in 2005, Junior began working with the FBI. It’s not clear how he became involved — prosecutors declined to say — but he told the court that he quickly became disenchanted with the lifestyle. He testified that he had brushes with the law and had been arrested on charges of grand larceny and disorderly conduct.

“That was not the life that I wanted to live,” he testified. “It was pointless, doing violent stuff for something that makes no sense.”

Junior had multiple FBI hand­lers that he worked with for years.

In check-ins by phone or in person, sometimes as often as once a week, Junior would share intelligence on the gang’s activities, including killings, he said. He was prohibited from engaging in any violence himself, but with special permission, he was allowed to take money from gang activities involving drugs, prostitution and extortion.

In 2006, Junior testified in the same federal court in the trial of Wilfredo Montoya Baires, a member of MS-13 who fatally shot another member. Baires was convicted of murder and sentenced to life in prison.

The Justice Department has paid Junior roughly $42,000 for his services over the past decade, including about $12,000 for expenses that included payments to fellow gang members, according to his testimony. The FBI has also filed paperwork to help him become a legal resident in the United States, he testified.

Amid all that, Junior maintained the guise of an MS-13 member. In 2012, he was even promoted to “first word,” or leader, of the Silvas clique, he said.

Attorneys for the defendants questioned how Junior was able to move up within the gang without participating in violence or other crimes. Junior said he succeeded by “knowing how to talk to them” and “buying time” to find ways to avoid criminal activity.

“It seems like you were buying time for 12 years,” said Manuel E. Leiva, an attorney for one of the defendants.

“Twelve years doesn’t mean I was in the streets 24/7,” Junior said.

Defense attorneys also said that Junior was motivated to exaggerate to the FBI because he needed money and help with his immigration status. They pointed to his attempts to become a legal resident as well as a 2015 bankruptcy petition Junior filed, in which he told a court that he was unemployed.

Under questions from Cerna’s attorney, Junior admitted that he lied about his employment status. In earlier testimony, he said he had always had a full-time job while working as an FBI informant.

Junior told jurors that he became involved in the investigation of the slayings after hearing in late 2013 that the defendants had killed Nelson Omar Quintanilla Trujillo, a member they believed was cooperating with police.

Junior said the FBI gave him a cellphone that automatically recorded all calls and texts. He used that phone for close to a year.

He also wore body wires to the monthly “general meetings” the gang held in hotels and parks in Woodbridge.

In those recordings, prosecutors said, gang members told Junior how they killed Trujillo and buried him in Holmes Run Stream Valley Park in Fairfax County.

They also told him how, in March 2014, they stabbed and decapitated another gang member, Gerson Adoni Martinez Aguilar, and buried him nearby, according to prosecutors. In later calls, prosecutors said, gang members told Junior about how they shot a man named Julio Urrutia in Alexandria.

In court, prosecutors asked Junior to interpret transcripts of the recordings translated by FBI contractors. Junior said multiple gang members took credit for the killings.

Defense attorneys, however, contended that Junior couldn’t be trusted to accurately interpret the exchanges, some of which took place more than two years ago.

Prosecutors said the afternoon of May 15, 2014, marked a breakthrough in the case. In a series of phone calls, Cerna had agreed to take Junior that day to visit a hill in Holmes Run park where Cerna’s clique allegedly buried Trujillo and Aguilar.

“It’s like a small incline,” Cerna told Junior, according to a transcript of a call presented in court. “It takes some effort to climb.”

Junior had met two FBI agents near the park earlier. They searched him and confiscated a marijuana joint he said he planned to smoke with Cerna. Then they outfitted him with an audio and video recording device.

Junior testified that he met Cerna at a nearby McDonald’s and that the two spent about 30 minutes trekking to what Junior said were the grave sites.

Prosecutors played video in court that showed a young man they said was Cerna, in plaid shorts and a dark polo shirt, brushing leaves and dirt aside with a stick. Junior can be heard breathing heavily and speaking in Spanish with Cerna, who, according to testimony, described how gang members poured acid on Trujillo’s grave to dissolve his remains.

Junior said he drove Cerna home and immediately went back to show FBI agents the site. In the following days, prosecutors said, authorities returned with cadaver-sniffing dogs and excavated the two bodies.

Authorities have since taken steps to protect some of Junior’s family members, he testified. Assistant U.S. Attorney Julia Martinez, the lead prosecutor in the case, asked Junior why he risked his life to locate the remains of the two men.

“It just didn’t feel right. I had to do something,” he said. “The FBI can only do so much.”