Vance Boelter, the man accused of assassinating a Democratic Minnesota state representative and shooting a state senator on Sunday, acquired the addresses of his victims and other alleged targets by using information collected by online data brokers, according to court documents obtained by Politico.

According to the report, police found the names of 11 registered data brokers written in a notebook that was recovered from Boelter’s vehicle. He also allegedly wrote, “most property records in America are public” in the notebook. It was previously reported that police found a list of other state and federal lawmakers in his truck, along with their addresses. It now seems those data brokers—which collect and sell personal information including addresses, phone numbers, email addresses, and possible relatives—were likely used by Boelter to identify the homes of his victims and other potential targets.

“Boelter stalked his victims like prey,” acting US attorney Joseph Thompson alleged at a press conference on Monday. “He researched his victims and their families. He used the internet and other tools to find their addresses and names, the names of their family members.” During the conference, Thompson also indicated that Boelter staked out the homes of his victims and surveilled them before allegedly carrying out his attack. Boelter has been charged with a total of six counts, including multiple counts of second-degree murder, per Wired.

In response to the revelation that Boelter allegedly used data brokers to target and ultimately murder Melissa Hortman and her husband Mark Hortman, two U.S. senators have advocated for a renewed effort to regulate the companies. “I have long advocated for data privacy for everyone, including the residences of lawmakers, and I have encountered resistance in the past. Maybe these horrific murders will change the sentiment within Congress,” Minnesota Senator Amy Klobuchar told Politico.

Klobuchar sponsored an amendment to the National Defense Authorization Act that would have allowed federal officials to remove their personal information from online databases. That amendment, which failed to pass, likely would not have protected the Hortman family, as it did not include protections for state-level lawmakers. Likewise, it would not have protected abortion providers who were also reportedly mentioned in Boetler’s hit list.

Senator Ron Wyden of Oregon also spoke out against the availability of personal information for sale by data brokers, and he is reportedly working on legislation to address it, per Politico. “Congress doesn’t need any more proof that people are being killed based on data for sale to anyone with a credit card. Every single American’s safety is at risk until Congress cracks down on this sleazy industry,” Wyden said in a statement to Politico.

The apparently politically motivated assassination allegedly carried out by Boelter is not the first instance of data brokers being used to facilitate an attack. In 2020, an attacker showed up at the home of District Judge Esther Salas and opened fire on her son and husband, killing the son. The alleged killer was also reportedly targeting Supreme Court Justice Sonia Sotomayor. In response to the attack, Congress passed a law barring data brokers from reselling federal judges’ personally identifiable information. But those protections do not extend to lawmakers, nor to private citizens who are also potential victims of stalking, abuse, and violence, without the headlines to accompany it or raise alarm bells.