He had been hiding in a hotel parking lot after fleeing from officers and was arrested at 2:30 a.m., according to an affidavit provided to The Washington Post by county prosecutors.
Alaniz said it is believed all four women were sex workers. Ortiz reportedly told investigators that he picked Ramirez up in the area of San Bernardo Avenue on September 3, then drove outside the city limits, according to the affidavit.
The woman was in a truck with a man named "David" and the two started talking about one of the prostitutes who had gone missing a week earlier.
At least two were United States citizens; the nationalities of the others were not known, he said.
"We have probable cause to believe that he is responsible for this series of murders, which I would qualify as a serial murderer", Alaniz told reporters at a news conference, the Houston Chronicle reported.
Investigators discovered two more bodies, one on Friday and one on Saturday in a remote area along Interstate Route 35.
Juan David Oritz has been charged over the murder of four women.
It is alleged the killings happened over a 10-day period, and Webb County District Attorney Isidro Alaniz said Ortiz continued working as normal throughout that time.
The federal agency issued a statement offering its "sincerest condolences" to the victims' families and saying criminal activity by its employees is not tolerated.
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"When she tried to escape from him at a gas station that's when she ran into a (state) trooper", he said.
Police say he confessed to four killings, but refrain from revealing details of the attacks.
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"I extend my condolences to the families of the victims during this hard time". The border city about 235 kilometres southwest of San Antonio.
The Texas Rangers and the Webb County Sheriff's Office are investigating the murders, authorities said.
Andrew Meehan, a US Customs and Border Protection spokesperson, said in a statement to CNN that the agency is fully cooperating with investigators.
Alaniz says authorities are still trying to determine a motive for the killings.
"Laredo is not the sleepy town that we grew up in", Alaniz said.