(KSTP) – Minnesota’s Bureau of Criminal Apprehension shared on Thursday that agents are still piecing together the fatal shooting of three Burnsville first responders, including how the suspect obtained his firearms.

On Sunday, Burnsville Police officers Paul Elmstrand, Matthew Ruge, and firefighter/paramedic Adam Finseth were killed when their SWAT team came under fire on a call.

One question that remains: How did the shooter get his weapons?

Court records indicate the suspect, who later took his own life, wasn’t legally allowed to have the guns due to his criminal record.

On Friday, 5 EYEWITNESS NEWS checked at the courts, and no new search warrants or documents were filed in the investigation.

Federal agent explains how any firearm can be traced

In 2022, Minnesota law enforcement asked the federal Bureau of Alcohol, Firearms, Tobacco and Explosives to help trace 5,327 firearms that were recovered as part of a variety of investigations, ATF data shows.

“We’re focused as an agency on addressing violent firearm offenses,” said Travis Riddle, special agent in charge of the ATF’s St. Paul field office. “Not only the drivers of violence but also the individuals who are providing the firearms to these suspects that are committing these acts of violence.”

5 EYEWITNESS NEWS spoke with Riddle to learn how gun traces can take place but did not discuss anything related to Burnsville.

The ATF can help local and state law enforcement trace firearms and learn about their past and who had them through their e-Trace program.

Details about the firearm are entered by law enforcement and reviewed by federal investigators who have access to records.

The tracing process includes paper searches of documents. Authorities can look to see the movement of the gun, from its first sale by a manufacturer or importer through the distribution chain, including the first retail purchaser. The ATF said tracing a gun in any case can help identify potential traffickers, whether licensed or unlicensed sellers.

If an investigation determines there was a straw purchaser involved, authorities try to find that person, since it’s a crime.

RELATED: BCA fields new mobile crime lab to speed up shooting investigations

The software takes images of ballistic evidence from shooting scenes and recovered guns.

Then, it compares it to what’s already in the system, to try and provide clues in an investigation.