A US Border Patrol agent in Texas has pleaded not guilty to capital murder and other charges in the September killings of four women who prosecutors say were sex workers.

Juan David Ortiz, a 10-year veteran of US Customs and Border Patrol, entered the pleas on Thursday before a judge in the border city of Laredo. He is accused of killing Melissa Ramirez, Claudine Luera, Guiselda Alicia Cantu and Janelle Ortiz.

Each was shot in the head and left along rural Laredo-area roads. One died of blunt force trauma after being shot.

Ortiz was apprehended after a fifth woman escaped and located a state trooper, according to authorities.

Webb County District Attorney Isidro Alaniz has said he'll seek the death penalty if Ortiz is convicted of capital murder.

"He deemed them to be the scum of the earth, and he wanted to move forward and clean the streets of these types of people," Alaniz said last month.

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'Hunting the streets'

Alaniz contends that Ortiz told investigators he was "doing a service" by killing the women and that he didn't think law enforcement was doing enough to curb prostitution.

Joel Perez, Ortiz's lawyer, said his client, "like any other person charged with a crime, is presumed innocent".

Up until the agent's arrest, Alaniz said, Ortiz "would go about his daily activities like anybody here". "He appeared normal by all accounts and circumstances."

"At the nighttime, he was someone else, hunting the streets of San Bernardo for this community of people and arbitrarily deciding who he was going to kill next."

Yanira Resendez, a spokeswoman for the Webb County District Attorney's office, said the mother of one of the dead women screamed at Ortiz in Spanish as he was led out of the court in handcuffs. "Damn you, you damn murder," she yelled.