HOUSTON — A U.S. Border Patrol supervisor was charged Saturday with murder in the deaths of four female sex workers following what authorities called a two-week killing spree that ended when a fifth woman escaped from him at a Texas gas station and found help.

Webb County District Attorney Isidro Alaniz said in a tweet that Juan David Ortiz, 35, an intel supervisor for the Border Patrol, had been charged with four counts of murder as well as aggravated assault and unlawful restraint.

Ortiz, a 10-year Border Patrol veteran, was arrested early Saturday after the fifth woman escaped and found a state trooper. Ortiz fled and was found hiding in a truck in a hotel parking lot in Laredo, about 145 miles (235 kilometers) southwest of San Antonio.

“We do consider this to be a serial killer,” Alaniz said.

Alaniz told The Texas Tribune that after Ortiz picked up the fifth woman she quickly realized that she was in danger.

“When she tried to escape from him at a gas station that’s when she ran into a (state) trooper,” Alaniz said.

He said that authorities believe Ortiz had killed all four women since Sept. 3. The names of the victims were not immediately released. Alaniz said two of them were U.S. citizens but the nationalities of the other two were not yet known.

“The manner in which they were killed is similar in all the cases from the evidence,” said Alaniz. He declined to say how they were killed.

Alaniz said investigators are still trying to determine a motive for the killings. Authorities believe he acted alone.

“It’s interesting that he would be observing and watching as law enforcement was looking for the killer, that he would be reporting to work every day like normal,” Alaniz said.

U.S. Customs and Border Protection issued a statement saying that it was fully cooperating with the investigation.

“Our sincerest condolences go out to the victims’ family and friends. While it is CBP policy to not comment on the details of an ongoing investigation, criminal action by our employees is not, and will not be tolerated,” the agency said..

The Texas Department of Public Safety, whose Texas Rangers are investigating, did not return several messages seeking comment.