LAREDO, Texas – Juan David Ortiz, a U.S. Border Patrol agent accused of murdering four women in the area last month, probably used his government-issued, semiautomatic weapon to carry out the killings, according to law enforcement officials.

Police found .40-caliber casings at the scene of each of the murders, consistent with the agency-issued .40-caliber HK P2000 semiautomatic pistol found in Ortiz’s truck when he was arrested Sept. 15, said Isidro Alaniz, district attorney for Webb and Zapata Counties, in an exclusive interview with USA TODAY.

The handgun and casings, as well as one projectile found at the scene of the last attack, were being examined at the Texas Department of Public Safety’s crime lab in Laredo to establish a precise match, he said.

Ortiz, 35, was arrested after a woman fled from his truck and alerted police. He was charged with four counts of murder and held at the Webb County Jail on $2.5 million bond. Officials described him as a "serial" killer.

“Certainly, right now, circumstantially, everything matches up,” Alaniz said of the murder weapon. “The scientific evidence will confirm what we believe to be true: that they came from the HK.”

Authorities alleged that Ortiz picked up the four women – Melissa Ramirez, 29; Claudine Ann Luera, 42; Griselda Alicia Hernandez, 35; and Nikki Enriquez, 28 – from Sept. 3 to 14, drove them to remote locations around Webb County and killed them each with gunshots to the head. All the women were sex workers who congregated around a street in Laredo known for sex and drug trade.

Ortiz was arrested hiding in the parking lot of a Laredo hotel and confessed to the murders, police said.

Investigators suspect he used his position as an intelligence supervisor in U.S. Customs and Border Protection's Laredo Sector to monitor the murder investigation as it circled in on him and stay ahead of detectives.

Webb County Sheriff’s investigators often share information about murders with other agencies active in the area, such as the DPS and Border Patrol – information that probably crossed Ortiz’s desk, Webb County Sheriff Chief Federico Garza said.

After Ramirez’s murder Sept. 3, investigators wrongly looked for a black truck or black Cadillac in connection with the killing, a misstep Ortiz probably followed closely, Garza said. Ortiz drove a white Dodge Ram truck.

“We strongly do believe that Ortiz was ahead of us because of the information that we were providing,” Garza said. “He knew what we were looking for. He knew that we were off him and looking in other areas. He knew all these things.”

The revelations of the weapon probably used in the shootings and Ortiz's use of his position as intel supervisor run counter to statements by CBP officials that denied the suspect used agency resources to allegedly commit the crimes.

"At this time, we have no reason to believe that the alleged perpetrator was acting in any official capacity or was on duty at the time of the crimes," CBP Commissioner Kevin McAleenan wrote last month to members of Congress.

The commissioner was responding to a letter written by 18 members of the House of Representatives – all Democrats, most from border states – requesting detailed information about the crimes Ortiz is charged with, whether he was on duty or used government resources while allegedly committing them and whether the agency failed to identify any changes in his conduct.

U.S. Rep. Henry Cuellar, D-Texas, whose border district includes the area where the attacks occurred, dispatched a letter to the agency asking for an analysis of CBP’s hiring practices.

Carla Provost, chief of the Border Patrol, said during a visit to Laredo shortly after the killings that Ortiz was placed on indefinite, unpaid suspension and that her office would assist law enforcement officials leading the investigation.

CBP, which includes the Border Patrol, has been plagued by charges against its agents for years, including arrests for alleged murders, cooperation with drug cartels and a wide variety of other crimes. A report by the Government Accountability Office stated that 20,333 misconduct cases – including criminal offenses – were brought against employees of CBP from 2014 through 2016.

Garza and other law enforcement officials stressed that Ortiz’s arrest shouldn’t cloud the agency’s important work along the border.

“This matter we speak of doesn’t reflect on any of the hardworking men and women of Border Patrol, sheriff’s office, police department, federal agencies – all of us,” he said.

However, the attack will probably leave a stain on law enforcement, Garza said.

“We’re here to serve and protect our community,” he said. “When something like this happens, it’s a black eye on all of us.”

Follow Jervis on Twitter: @MrRJervis.