A US Border Patrol agent who confessed to killing four sex workers because he wanted to "clean up the streets" of his Texas home town has been indicted for capital murder by a grand jury.

Key points: Prosecutors say Oritz was a family man by day but "hunted" sex workers by night

Prosecutors say Oritz was a family man by day but "hunted" sex workers by night Ortiz believed police did not do enough to curb prostitution

Ortiz believed police did not do enough to curb prostitution A woman Oritz tried to kill escaped and helped investigators track him down

Prosecutors will seek the death penalty for the September murders, with Webb County district attorney Isidro Alaniz saying evidence showed Juan David Ortiz killed the women "in a cold, callous and calculating way".

"The scheme in this case, from Ortiz's own words, was to clean up the streets of Laredo by targeting this community of individuals who he perceived to be disposable, that no-one would miss and that he did not give value to," Mr Alaniz said.

Mr Alaniz said Ortiz, 35, believed law enforcement did not do enough to curb prostitution, so he was "doing a service" by killing the women.

A suspect can be charged with capital murder if they are suspected in more than one killing in the same scheme with an overarching motive, Mr Alaniz said.

Three of the women were shot dead while the fourth victim was also shot but died of blunt force trauma.

Mr Alaniz said the horrific nature of the killings and Ortiz's vigilante mentality were factors in his decision to pursue the death penalty.

Soon after Ortiz was arrested, Mr Alaniz described his crimes as a two-week serial killing spree.

Ortiz, who has been held on murder charges in the Webb County jail on a $US2.5 million ($3.46 million) bond since his arrest on September 15, presents a clear danger to society, Mr Alaniz said.

The Border Patrol intel supervisor and Navy veteran seemed to be living a typical suburban life with his wife and two children when the killings occurred.

He was only arrested after a victim was able to escape him and ask a state trooper for help.

"By day, he was a family man. The evidence shows that he was a supervisor, that he would go about his daily activities like anybody here. He appeared normal by all accounts and circumstances," Mr Alaniz said.

"At the night time, he was somebody else — hunting the streets … for this community of people and arbitrarily deciding who he was going to kill next."

Two of the victims were US citizens but the nationalities of the other two were not yet known. ( The Laredo Morning Times via AP: Danny Zaragoza )

Who were the victims in Ortiz's serial-killing spree?

Mr Alaniz said Ortiz knew some of the victims but he would not elaborate on what kind of relationship they had.

Melissa Ramirez, 29, was slain on September 3, and 42-year-old Claudine Luera was killed 10 days later.

On September 14 Ortiz picked up Erika Pena, who told investigators he acted oddly when she brought up Ms Ramirez's killing and later pointed a gun at her in a gas station, according to court documents.

Ms Pena said Ortiz grabbed her shirt as she tried to exit his truck, but she pulled it off and ran, finding a state trooper who was refuelling his vehicle.

Ortiz fled and then picked up and killed his last two victims, 35-year-old Guiselda Alicia Cantu and 28-year-old Janelle Ortiz, he later told investigators.

With Ms Pena's help authorities were able to track Ortiz to a hotel parking garage where he was arrested.

"I believe that if Erika Pena would not have escaped that day that there would be more victims right now in this case," Mr Alaniz said.

Ortiz also was indicted on Wednesday (local time) on charges of aggravated assault with a deadly weapon and unlawful restraint in the attack on Ms Pena, and a charge of evading arrest or detention.

The Border Patrol placed Ortiz, who had worked for them for 10 years, on indefinite, unpaid suspension after his arrest.

ABC/AP