A game at the Barry Farm public housing project, which prosecutors say was the home turf of a violent drug-running gang led by Mark Pray. (Jonathan Newton/Washington Post)

When Crystal Washington checked out of her halfway house and stepped into a gray April morning, the gunman was ready.

He silently slipped behind Washington as she crossed a busy street at rush hour, then he pulled out a semiautomatic pistol and fired six shots at point-blank range, killing the 44-year-old mother of four.

It would not take long for authorities to piece together a motive — Washington, a recovering drug addict, was a key prosecution witness at the looming trial of the gunman’s boss, an alleged Southeast Washington narcotics kingpin.

The slaying in 2009 soon sparked an undercover FBI investigation straight out of the HBO series “The Wire” that would lead to charges against 13 alleged members of a Southeast drug gang. On Feb. 1, the trial of three of those people began in the District’s federal court. Six defendants have pleaded guilty; the dispositions of four others could not be determined from court records.

The investigation provides a rare window into an organization that dominated a vibrant open-air drug market in Southeast, a gang that stopped at nothing to protect its turf. The case also helps explain the city’s stubborn level of violence. Although homicides have plummeted in the District (last year, city police tallied 108 killings, nearly half the number recorded in 2004), authorities say they are still battling an intractable cycle that fuels the city’s murders — territorial disputes and retaliation for snubs and other acts of violence.

In the case of the Southeast gang, prosecutors say, its ringleader, Mark Pray, thought little of conspiring to kill Washington, a woman he had known for years. A few months later, prosecutors say, Pray and his chief “enforcer” — the same man accused of shooting Washington — agreed to eliminate a rival dealer because that man’s brother had shot one of Pray’s lieutenants.

And, in 2008, angered that another dealer had disrespected one of his associates, Pray participated in a drive-by shooting: He popped out of the sun roof of a speeding Cadillac, whipped out two semiautomatic pistols and littered a major street with 27 bullets that riddled another car and killed his intended target, prosecutors say.

“This is a case about greed,” federal prosecutor Matthew Cohen of the District’s U.S. attorney’s office told jurors. “It’s a case about the greed that drove the defendants to sell drugs in this community. . . . These co-conspirators took human life to help each other. They were part of a group with a common purpose that committed a series of acts that included drug dealing and murder.”

The three defendants, Pray, 31, his alleged enforcer, Alonzo Marlow, 30, and Kenneth Benbow, 31, have denied involvement in any homicides. Their attorneys have argued to jurors that the government’s evidence is ambiguous and not as strong as prosecutors have said. They also have criticized what they have called sloppy police work.

According to court papers, prosecutors and testimony in a trial that is expected to last eight weeks, the investigation can be traced to Washington’s rowhouse in the Barry Farm public housing project, a cluster of drab, white-and-beige two-story units nestled near Interstate 295 and south of Suitland Parkway. The neighborhood has long endured the violence and social ills associated with being an open-air drug market — one that prosecutors say Pray and his organization had come to dominate by 2005.

To hide his product from police, Pray took over other people’s houses, and that is what led him to Washington, prosecutors say. In exchange for drugs, prosecutors say, Washington allowed Pray and his cohorts to pack, process and store PCP, crack cocaine and marijuana in her home in the 1200 block of Eaton Road. He and his dealers also hid guns in the house — in fact, Washington’s 12-year-old son even found two rifles stashed in the laundry room, the mother later told a grand jury.

One evening in September 2006, after returning home from the grocery store, Washington paid Pray $80 for a cigarette dipped in PCP and went upstairs to get high, court papers show.

Within hours, prosecutors say, police responded to an anonymous call from a woman complaining about “unwanted guests” in the house; authorities would later determine that the call was placed by Washington’s teenage daughter.

When officers arrived, they said they found Pray sitting at a kitchen table that was covered with 14 half-ounce vials of PCP, 36 Ziploc bags of marijuana, 4.8 grams of cocaine, a digital scale and $230 in cash. Police also recovered two rifles and a semiautomatic handgun with rubber bands strapped to its grip, an effort to prevent fingerprints from being left on the weapon.

Pray, Washington and several others were arrested and charged. Washington eventually agreed to cooperate with authorities in exchange for a lesser sentence. As a condition of her release, Washington lived in the halfway house in the 1400 block of G Street NE.

After her slaying just three days before Pray’s trial, prosecutors say, the case evaporated because there was no way for them to prove that the guns and drugs belonged to the alleged dealer. As investigators dug into the killing, they got lucky and discovered that Marlow was wearing a court-ordered ankle bracelet that used a Global Positioning System device to track his location. The bracelet tracked him to within just a few feet of Washington when she was slain, according to prosecutors, who allege that Pray conspired to have her executed.

Within weeks of Washington’s death, prosecutors say, federal agents launched an investigation of Pray and his gang. They sent undercover informants to purchase PCP and other drugs — capturing the deals on video. Agents also tapped Pray’s phones, an investigative technique that allowed them to arrest his alleged heroin supplier in Baltimore and to better understand how his organization worked — after agents deciphered the dealers’ code. Apparently worried that their calls were being monitored, the dealers routinely called Barry Farm “Noya,” PCP “munga” and cocaine “Joe.” Handguns were “pocket rockets,” and Pray’s flashy black Lexus was known as “the Bat,” prosecutors say.

Like many street gangs, authorities say, Pray’s organization had to fight for its turf. In July 2009, one of Pray’s trusted lieutenants was wounded in Barry Farm by a rival. When Marlow, incarcerated in an unrelated case, got out of prison Dec. 30, prosecutors say, one of the first people he called was Pray. “Help is on the way,” he told him, according to prosecutors.

Looking to retaliate, Marlow began hunting for the assailant, only to learn that the man was in jail. Instead, Marlow turned his attention to the attacker’s brother, prosecutors say.

On the morning of Jan. 13, 2010, Marlow spotted the brother in Barry Farm and soon was on the phone with Pray, telling him that he was making a “work call,” prosecutors say. Slipping into code, they talked about killing the man, prosecutors say, and Pray recommended that Marlow wear a hooded sweatshirt so nobody would recognize him.

Fifty minutes later, the brother was dead.

Again, prosecutors say, Marlow blundered: He was wearing a GPS ankle bracelet.

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