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Michael Blackman kept firing, even as the Chicago police officers riddled his body with gunfire.

The Sept. 21 shootout marked the end of a violent spree that began three days earlier, when Blackman shot a woman in the back, apparently at random, in the middle of the day, according to charges. His toll now included shooting Chicago police officer Adam Wazny three times, and narrowly missing several others, police say.

Hidden inside Blackman's 9-millimeter pistol was a clue to a separate crime. It came in the form of a constellation of tiny imperfections, the missing piece to a brutal mystery more than 400 miles away in Minnesota.

The system used to decode these tiny markings is called National Integrated Ballistic Information Network, or NIBIN for short, which forensic scientists use to analyze the unique signatures guns leave on the casing of ammunition.

"We can trace the life of a gun without having the bodies," said Amber Brennan, assistant U.S. attorney in Minnesota.

Unlike a person, Brennan said, a gun can't lie.

In the Twin Cities, NIBIN is becoming a pivotal investigative tool to thwart a rising threat of gun violence and illegal firearms flooding the streets. In St. Paul, which is nearing a record number of homicides this year, mostly from gunfire, a new $750,000 federal grant will fund a NIBIN technician to trace the history of firearms. Hennepin County's gun lab will be getting its own NIBIN system next month.

Federal law enforcement agents say the technology illuminates how one gun can change hands and travel across cities and states, leaving a trail of dead or injured victims in its wake.

"It's very common for us to hear, 'This gun was used in another recent shooting,' " said Brennan.

This was the case with Blackman's gun.

Using the serial number, federal agents traced the 9mm to Bill's Gun Shop in Robbinsdale, where it had been bought by a woman named Sequana Cigolo. The markings on the casings matched spent shells found after a home invasion in northeast Minneapolis.

It was Cigolo's home.

And the victim was Cigolo's brother.

How to read a gun

Inside the Bureau of Criminal Apprehension's crime lab in St. Paul, in a long, narrow room lined with soundproof padding, sits a water tank resembling a giant metal coffin.

When police want to learn where a gun has been, they fire it into a slot on the side of the tank. Water slows the bullet and a net catches the shell casing intact, ready to be tested.

The NIBIN machine is less a spectacle; it could be mistaken for a large black printer. On a recent Wednesday, Ashley Fetch, a forensic scientist with the BCA, placed a shell case inside the machine, and NIBIN whirred to life. A computer monitor conjured images of the circular casing base. Using different types of lighting, the machine takes images of the casing to create a 3-D rendering, explained Fetch.

The image is then transmitted to Alabama, where specially trained analysts for the Bureau of Alcohol, Tobacco, Firearms and Explosives review potential matches. They will look for a marking left by the "firing pin," a small metal piece that presses against the primer of a casing when a gun fires. Another imprint is stained on the "breech block," the plate that closes when a gun is fired. Together, these two mechanisms leave a micro-impression.

If the impression matches a casing recovered from another crime scene, it creates a lead. When forensic scientists confirm the match, it's a hit.

Some have questioned the accuracy of a NIBIN hit, and whether an imprint is truly unique. But prosecutors in Minnesota say they don't use NIBIN to charge a suspect on its own. They describe it as an investigative tool designed to connect dots they may not have otherwise thought were related.

"NIBIN will narrow down that universe," said Assistant U.S. Attorney Jeffrey Paulsen. "But you can't take that to court. You've gotta have the expert come in and ... do the physical matchup, just like a fingerprint."

The technology isn't new; police have been tracing guns through some form of ballistic comparison technology since the 1920s. But law enforcement has been using NIBIN more over the past few years, said Jonathan Ortiz, assistant special agent in charge for St. Paul's ATF office. "Police agencies are seeing the value in it, and they want to participate."

Over the past three years, ATF has tested more than 11,000 shell casings recovered from either the BCA's machine or one used by Minneapolis police, resulting in 1,824 hits, according to data provided by the ATF.

One of those was the SCCY CPX-2, 9mm semi-automatic pistol Blackman allegedly used to shoot a police officer.

Connecting the dots

In 2015, for the first time, federal prosecutors in Minnesota used NIBIN to show how members of a gang share guns.

The case involved a gang war in north Minneapolis between allies 1-9 Block Dipset and Stick Up Boys and their rivals, Taliban and Young 'N Thuggin'. Police raided a 1-9 house and found a stash of guns. Using NIBIN analysis, investigators linked one to a pair of shootings of Taliban houses.

"In the olden days, we'd kind of throw up our hands and say, 'Welp, there's no DNA on these guns. It wasn't in anybody's pocket. I guess nobody's responsible,' " said Paulsen, who prosecuted the case. "Under this theory, it's just the opposite: Everyone's responsible."

Paulsen charged 11 gang members in a conspiracy case involving buying firearms illegally and passing them around to use in the gang feud.

NIBIN similarly helped connect the dots in the investigation of Blackman's gun.

On Sept. 24, three days after the standoff with Chicago police, agents from the ATF knocked on Cigolo's door.

The 38-year-old had no serious criminal history, meaning she could legally buy a gun — presuming she's buying it for herself. Cigolo told the agents she'd been approached by a man named Jason Winston, the cousin of her ex-boyfriend, according to a summary of the conversation in court documents.

Winston did have a felony record, precluding him from possessing a firearm, and paid Cigolo $250 to buy it for him, according to court records.

When the agents searched Winston's home later that day, they found a safe containing 9mm bullets and the gun's original safety lock with a matching serial number, according to the court documents. Winston admitted to paying Cigolo for the gun, and said he'd sold it to his cousin — Cigolo's ex-boyfriend — a man named Michael James Gresham, in July.

Gresham had a violent past. Court records show he was convicted of second-degree assault for shooting his brother in 2015.

On July 27, two weeks after Winston allegedly sold him the gun, and on the heels of his breakup with Cigolo, Gresham stole Cigolo's car, according to Hennepin County court records. The next day, he came to Cigolo's house under the guise of returning her belongings.

Prosecutors say Gresham walked directly up to Cigolo's brother's room and found him lying in bed. Then he fired six shots at his head, according to charges.

At the last moment, the brother rolled off the bed, catching only one of the bullets in the leg. Gresham climbed back into Cigolo's car and sped away, according to charges.

He drove to Chicago.

When police later arrested him, the gun was gone.

Aftermath of three shootings

Ortiz says NIBIN was critical to tying the Chicago case to Minnesota. Without it, "we would never have found out about the other shootings," he said.

The U.S. Attorney's Office has charged Cigolo and Winston with illegally purchasing a gun. They appeared in federal court in late October.

Gresham is now in the Hennepin County jail awaiting a trial scheduled for early next year. He faces charges of second-degree assault and illegally possessing a firearm, both felonies. If convicted, he could be sentenced to between eight and 22 years in prison.

After several surgeries and a month in the hospital, Chicago officer Wazny was recently allowed to go home. In a visit to Chicago in late October, President Donald Trump commended Wazny for his "tremendous bravery," calling the shooting part of a "dangerous trend of criminals assaulting police."

The woman Blackman shot in the back survived the attack, but the bullet left her paralyzed, according to prosecutors.

Blackman survived, too. He was taken to intensive care, and has since been moved to the Cook County jail. He faces five counts of attempted murder, and has been denied bail.