Gerson Adoni Martinez Aguilar was in trouble with fellow members of his gang, MS-13. He owed them $600 from drug sales, and he was accused of having sex with another member’s girlfriend.

In March 2014, a half-dozen gang members allegedly lured Aguilar to a park in Fairfax County and told him he had to face a beating, the customary punishment for breaking MS-13 rules. But instead of beating him, they killed him, stabbing him repeatedly with knives and machetes, then chopping off his head, according to prosecutors.

The slaying is part of a sweeping criminal case that went to trial last week, accusing people in the gang of carrying out three murders and an attempted murder in Northern Virginia between October 2013 and June 2014. Seven alleged MS-13 members are being tried in federal court in Alexandria and could face life in prison. Another six have entered into plea agreements, with conditions that they work with prosecutors.

The proceedings are expected to last six to eight weeks in front of U.S. District Judge Gerald Bruce Lee, during which time the court will be under increased security. A jury will hear testimony from leading gang experts, law enforcement agents and former gang members, some of whom participated in the alleged crimes.

The trial comes at a time when the Justice Department and local police say MS-13 has sought to reestablish itself in the Washington region after several years of relative quiet. Also known as Mara Salvatrucha, MS-13 has long been considered one of the region’s most dangerous street gangs, with members linked to brutal murders, beatings, drug trafficking and prostitution.

Seven male defendants in their early to late 20s, along with their attorneys, packed a top-floor courtroom in the U.S. District Court for the Eastern District of Virginia on Wednesday for the start of trial. Most wore light, collared shirts with dark pants and had close-cropped hair. Three defendants wore headsets to hear the proceedings interpreted in Spanish.

In opening statements, prosecutors said the defendants all belonged to a prominent MS-13 clique known as Park View Locos Salvatruchas that operates in Northern Virginia. Prosecutors said gang members killed to bolster their reputations and climb ranks within the gang.

“MS-13 glorifies violence above all else,” Assistant U.S. Attorney Julia Martinez said. “You must commit murder or another act of violence. Gang members did just that.”

The government’s case will hinge on testimony from a confidential informant and several defendants who have agreed to cooperate with prosecutors in exchange for potentially lighter sentences.

The informant, identified only as “Junior,” has worked with the FBI for more than a decade. Over the course of nearly a year, prosecutors said, Junior infiltrated the Park View Locos Salvatruchas, secretly recording cellphone conversations and gang meetings. He was not present for any killings, but gang members frequently bragged to him, prosecutors said.

Also scheduled to take the stand are some of the MS-13 members who prosecutors say helped commit the murders but took plea deals.

Defense attorneys are seeking to play down their clients’ roles in the case by questioning the credibility of those witnesses. In opening statements, they argued the defendants who pleaded guilty could not be trusted to give honest testimony, saying they had every reason to pin the blame on other members.

“The government has really scraped the bottom of the barrel as far as witnesses go,” said Manuel E. Leiva, an attorney for Jose Lopez Torres, who faces murder and attempted murder charges. “They have so much blood on their hands. They have every motive in the world to cooperate and fabricate.”

Defense attorneys said one cooperating defendant, Jose Del Cid, took part in nearly every crime alleged in the case. They said he is known to have a history of violence both in the United States and while living in El Salvador.

“A person who spent almost a lifetime killing is now a star witness for the government,” said Amy Austin, an attorney for Alvin Gaitan Benitez, who is charged with helping to slay Aguilar.

An attorney for Del Cid did not immediately respond to a request for comment.

Prosecutors unsealed a nine-count indictment in the case in October 2014, charging 13 people with murder, conspiracy, attempted murder and firearms violations.

The indictment offered few specifics about the alleged offenses. But in opening statements, prosecutors laid out the charges in vivid detail.

Martinez described Aguilar as a minor player in the gang who sold drugs and was having sex with the girlfriend of a gang member who was incarcerated. His behavior angered the gang enough that seven members plotted to kill him, she said.

When gang members killed Aguilar, Martinez said, they put his severed head in a canvas bag and buried his body in Holmes Run Stream Valley Park in Fairfax County.

It was not the first time they had carried out such a plan, she said.

Six months earlier, the clique’s leaders allegedly ordered the killing of a fellow member, identified by prosecutors only as “Peligroso” or D.F., who they believed was trying to leave MS-13.

On an evening in October 2013, gang members drove to Gar-Field High School in Woodbridge, Virginia, to intercept the man as he was leaving night classes. They were carrying two machetes and a sawed-off shotgun, records show. But when they arrived, police were there waiting for them. An informant had alerted authorities a day earlier.

“Peligroso was fortunate,” Martinez said. “No one came forward and told police that these other victims were going to die.”

The gang thought one of its members, Nelson Omar Quintanilla Trujillo, was the “rat” who told police about the plot, Martinez said. A week after the failed killing of Peligroso, four gang members allegedly brought Trujillo to Holmes Run Park, saying they were having a gang meeting. There, they stabbed him in the stomach with knives and slashed his face with a machete, killing him, Martinez said.

After burying his body in the park, two gang members returned later to rebury it in a less conspicuous location, fearing someone would find it, prosecutors said. Authorities were only able to discover Trujillo’s and Aguilar’s bodies after Junior, the FBI informant, persuaded a gang member to show him where they were hidden, Martinez said.

Martinez added that the gang was mistaken about Trujillo going to police.

In the third alleged killing, prosecutors claim a small group of gang members went out in Alexandria in June 2014 searching for rival gang members to fight. One gang member, Jesus Alejandro Chavez, allegedly confronted Julio Urrutia as he was hanging out in front of an apartment complex. After an argument about Urrutia’s gang affiliations, Chavez allegedly shot Urrutia in the back of the neck, prosecutors said.

The case mirrors other indictments brought by the Justice Department in recent months. In January, prosecutors indicted 56 members of MS-13 in the Boston area on conspiracy, murder, drug trafficking and other charges. In Maryland, three members of the gang were indicted last summer on charges of murder and attempted murder. And in May last year, 37 gang members in Charlotte were indicted on similar charges.

Formed in Los Angeles in the 1980s and with leadership in El Salvador, the gang has spread throughout the United States, operating in the Washington area since the 1990s. The gang has an estimated 2,000 to 3,000 members in the region, gang experts say.

MS-13 gained notoriety more than 10 years ago after a series of bloody attacks in Northern Virginia.

In one incident in 2004, gang members chopped off a 16-year-old boy’s fingers with a machete. The following year, gang members killed a pregnant member after learning she was cooperating with prosecutors.

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