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.
US border agent pleads not guilty to killing 4 Texas women →
Alaniz says Ortiz said he wanted to "clean up the streets of Laredo" by killing the women.
The Latest: Prosecutor calls woman who escaped 'very brave' →