A U.S. Border Patrol supervisor accused of killing four Texas women and kidnapping a fifth was jailed Sunday on $2.5 million bond.

Juan David Ortiz, 35, was arrested early Saturday after he was found hiding in a truck parked outside a Laredo hotel. He is charged with four counts of murder, as well as of aggravated assault with a deadly weapon and unlawful restraint.

Webb County District Attorney Isidro Alaniz said Saturday that investigators "consider this to be a serial killer." He noted that all four of the dead had worked as prostitutes and all four were shot in the head and left in a road in the rural south Texas county.

Investigators said the violence took place over 11 days, beginning with the Sept. 4 discovery of the remains of 29-year-old Melissa Ramirez. According to a police affidavit, Ortiz said he killed Ramirez, a mother of two, the previous day.

A second woman, 42-year-old Claudine Anne Luera, was found shot and left in the road Thursday morning, badly injured but still alive, according to the affidavit.

The mother of five died at a hospital later that day.

On Friday, according to the affidavit, Ortiz picked up a woman named Erika Pena. She told police she struggled with Ortiz inside his truck, where he pointed a pistol at her, but that she was able to escape. She made it to a gas station where she found a state trooper whom she asked for help.

BORDER PATROL AGENT ARRESTED, ACCUSED OF KILLING 4 WOMEN AND KIDNAPPING A 5TH

According to the affidavit, Ortiz told investigators that after Pena ran off, he picked up his last two victims, whose identities have not yet been released by authorities.

Jail records don't list an attorney to speak for Ortiz, who had worked for Border Patrol for 10 years.

Alaniz said one of those killed was a transgender woman. At least two were U.S. citizens; the nationalities of the others were not known, he said. He said investigators were still working to determine a motive.

Ortiz was believed to have acted alone.

The federal agency issued a statement offering its "sincerest condolences" to the women's families and saying criminal activity by its employees is not tolerated.

The Texas Department of Public Safety, whose Texas Rangers are investigating, referred questions on the case to the Webb County Sheriff's Office. Sheriff Martin Cuellar did not return several messages seeking comment.

Fox News' Paulina Dedaj and The Associated Press contributed to this report.