Isaias Valdez Rios, a former soldier who was recruited as a bodyguard for the notorious kingpin, said he shot dead two members of the Zetas cartel, who had been brought to him by his guards. After Guzman shot them, he ordered that their bodies be burned.
“Put them in the bonfire. I don’t want any bones to remain,” the witness told the New York court, according to the Associated Press.
He also told the court that the Sinola cartel boss ordered his men to dig a grave before he shot another victim in a fit of rage.
When the gunfire stopped, the wounded man “was still gasping for air”. “That’s how we dropped in the hole and buried him.”
Guzmán is charged with 17 criminal counts, including drug trafficking, conspiring to murder rivals, money laundering and weapons offences. He has pleaded not guilty.
During the early- and mid-2000s, he was allegedly a boss of the Sinaloa cartel, the world's largest drug-trafficking operation, according to prosecuors.
They say say the enterprise imported tons of cocaine from South America into the US.
The 61-year-old has twice escaped from jail, most recently in 2015 when he fled from a maximum security prison in the Mexican city of Almoloya de Juárez, through a long tunnel.
He was captured and arrested for a third time in January 2016 and extradited to the US the same month.
He is being tried in the federal district court in the New York borough of Brooklyn. The trial, which began in mid-November, is expected to last four months.