[What you need to know to start the day: Get New York Today in your inbox.]When Edgar Galvan got divorced in El Paso, Tex., in 2003, he did what many others in a similar position might have done: look for a good time. To do so, Mr. Galvan rented a house across the Mexican border in El Paso’s sister city, Ciudad Juárez.But as he began frequenting the city’s raucous night clubs, Mr. Galvan — then only 26 — made poor choices about who he spent his time with. He became “party friends,” he said, with Antonio Marrufo, a bloodthirsty killer known as Jaguar, who would soon be tasked by the Sinaloa drug cartel with “cleansing” Juárez of its rivals. Within four years of meeting Jaguar, Mr. Galvan began working for him, receiving shipments of Mexican cocaine and marijuana at safe houses in El Paso and moving guns in the opposite direction.On Monday, Mr. Galvan, now 41, spoke about his brief stint in the smuggling trade at the drug trial of the longtime cartel leader, Joaquín Guzmán Loera, the kingpin known as El Chapo. So far, most of the witnesses at the federal trial in Brooklyn have been Mr. Guzmán’s lieutenants, suppliers or distributors — people who emerged from his organization’s managerial class. A luckless figure with a quiet voice and an unassuming manner, Mr. Galvan was the first witness who could be described as a minor worker bee.ImageAntonio Marrufo, nicknamed Jaguar, was an assassin for the Sinaloa drug cartel.CreditUnited States Attorney’s Office, Eastern District of New YorkIn an afternoon on the witness stand, Mr. Galvan told the jurors that he had never even met Mr. Guzmán and had only heard his voice one time — when he was in the room as Mr. Marrufo talked to the crime lord on the phone. At best, Mr. Galvan was a guy who knew a guy who knew the guy in charge.In that way, Mr. Galvan, who is serving a 24-year prison term on drug and weapons charges, was emblematic of the hundreds of minions who prosecutors say helped Mr. Guzmán turn the cartel into a multibillion-dollar operation that shipped huge loads of heroin, cocaine and marijuana into the United States on trains, planes, trucks, fishing boats and submarines.But in a trial where witnesses have routinely talked about illicit drug shipments that could weigh 200,000 kilograms or more, Mr. Galvan’s bulk weight shipped was paltry: In the three years before he was arrested, he claimed he moved only about 250 kilos of cocaine.It appeared from his testimony that he was brought to court largely to corroborate earlier descriptions by law enforcement officers of a seizure of 40 AK-47 rifles that took place in El Paso in 2010. Mr. Galvan said on Monday that the cartel needed the rifles to fight a bloody war at the time against a breakaway group called La Linea, which was founded by Mr. Guzmán’s ally-turned-rival, Vicente Carrillo Fuentes.At first, Mr. Galvan said he was leery of working for Mr. Marrufo — a frighteningly violent man. He told the jurors how one day his employer took him to a house in Ciudad Juárez that had a room with a white-tiled floor that sloped toward a drain.“That’s where he killed people,” Mr. Galvan said.But Mr. Galvan took the job of smuggling guns from Texas to Mexico — just as he had smuggled drugs the other way — because it was not easy to defy Mr. Marrufo.“Jaguar is not the kind of person who asks you questions,” he said. “He gives orders.”Though Mr. Guzmán’s empire mostly concerned itself with narcotics, American officials have long maintained that it also did a brisk trade in illegal guns, including military weapons like rocket-propelled grenade launchers and .50-caliber rifles. Last week, Vicente Zambada Niebla, the son of Mr. Guzmán’s partner and the cartel’s heir apparent, testified that 99 percent of the guns he purchased came from the United States.Mr. Galvan said he successfully moved four or five shipments of American weapons from El Paso to Juárez before misfortune struck on Jan. 13, 2010. On that day, the El Paso police raided one of his stash houses after trailing a suspicious Volkswagen Jetta.Before Christmas, prosecutors wheeled an evidence cart filled with the 40 seized rifles into the courtroom, delighting some jurors and terrifying others. But at the time of the raid, Mr. Galvan had his own problems: When Mr. Marrufo learned of the raid, he said, he was “really pissed.”“That was the last time I ever spoke to Jaguar,” Mr. Galvan said.