The killing of George Floyd on May 25 set in motion a massive and complex investigation only now coming into fuller view after the conviction of former police officer Derek Chauvin.

In a crowded office at Minneapolis police headquarters, state investigators first met with Chauvin to question him soon after the fatal encounter that would touch off weeks of protests and unrest worldwide. Chauvin declined to be interviewed by authorities, but by then the high-stakes investigation was growing in scope and urgency.

Hundreds of pages of records obtained by the Star Tribune provide a behind-the-scenes look at how the probe into Floyd's death frantically unfolded as Minnesota Bureau of Criminal Apprehension agents spent countless hours poring over video footage, interviewing witnesses and reviewing forensic and digital evidence.

They eventually pieced together a detailed timeline of the incident, starting from when Floyd walked into Cup Foods on the warm May evening to his takedown and arrest by police for allegedly passing a fake $20 bill. Their counterparts at the FBI were doing the same as news and harrowing bystander footage of Floyd's death stirred outrage around the globe.

The world's eyes were on Minneapolis again Tuesday when a jury found Chauvin guilty of all three counts of murder and manslaughter after deliberating for less than 10 hours. He is expected to be sentenced later this year.

But, as the first protesters took to the streets the day after Floyd's death last year, investigators from the BCA were already working against the clock to get to the bottom of what happened — and why.

By then many people had seen the cellphone footage of Floyd's arrest, showing him pleading to breathe as officers Chauvin, J. Alexander Kueng and Thomas Lane pinned him to the pavement — while a fourth officer, Tou Thao, stood guard — until he fell unconscious and died. All four were later fired and charged in Floyd's death.

BCA spokesperson Jill Oliveira declined to answer questions about the investigation, saying Chauvin's case remains active until appeals have been exhausted. A Minneapolis police spokesman declined to comment.

On the BCA's end, the investigation was run by senior special agent James Reyerson and the members of the agency's recently formed Force Investigations Unit, who like Reyerson brought years of experience investigating police shooting and in-custody deaths.

Reyerson got a call on the day Floyd died telling him to report to Room 108 at police headquarters downtown — an office that used to house most of the department's detective units, including homicide, robbery and assault. Minneapolis Police Department Chief Medaria Arradondo and most of his command staff had arrived, as had former police union president Lt. Bob Kroll. Chauvin was also there, coming from HCMC, where he had gone after Floyd was taken away in an ambulance.

Officers declined interviews

After conferring with his boss, Reyerson and another agency official met with Chauvin, Kueng and Lane to photograph the officers and collect their uniforms, guns and equipment. But the three officers declined to speak to investigators or give blood samples, on the advice of their union representatives. Meanwhile, another senior agency official went out to 38th and Chicago to oversee the collection of evidence at the scene, which hadn't yet been flooded with angry protesters demanding that the officers be fired.

The day after Floyd died, Reyerson went back to work. A former New York City cop and Drug Enforcement Administration agent, he had investigated other high-profile police killings in the city — names like Thurman Blevins, Travis Jordan and Justine Ruszczyk Damond that were instantly recognizable to social justice activists. But nothing compared to the Floyd case.

As the case wore on, massive demonstrations broke out in Minneapolis and beyond, with some elected officials and activists calling for fundamental changes in policing — only adding to the pressure on investigators. The BCA was also still wounded from criticism during the case of Mohamed Noor, when prosecutors openly accused the agency of mishandling certain elements of the investigation into the former officer's fatal shooting of Damond, an unarmed Australian woman who had called 911 to report a possible assault. During the trial, prosecutors argued that investigators failed to follow up with certain witnesses.

On May 29, the day after some protesters torched the Third Precinct police station, Reyerson was present when Chauvin was arrested at the office of his then-attorney and taken to BCA headquarters, where Chauvin again declined to be interviewed.

The weeks that followed were a whirlwind of interviews with eyewitnesses, Police Department officers of all ranks, medical experts and anonymous tipsters. Almost from the beginning, the BCA was flooded with scores of tips and leads, every one of which had to be painstakingly chased down.

A caller in Mexico claimed to be Floyd's mother, who had been dead for two years. A jailhouse informant contacted authorities to say that he had sold Floyd cocaine and methamphetamine. Someone else claimed to have a photo of one of the officers involved wearing a "White Lives Matter" hat. A woman said she remembered being pulled over by Chauvin in the 1990s and subjected to racist treatment, but when investigators ran her license through a database, they discovered that Chauvin hadn't yet joined the MPD at the time the stop was said to have happened.

Building a portrait

Without being able to question Chauvin, investigators worked to pull together a portrait of the fired officer through interviews with his former colleagues and even relatives of people killed in other police encounters in which Chauvin was involved, including speaking with the sister of Wayne Reyes, who was fatally shot in 2006 after a chase that ended when he pointed a sawed-off shotgun at officers.

Investigators reviewed Chauvin's personnel file, which summarized his employment history, psychological evaluations, and training and disciplinary records. They also scrutinized Chauvin's record as a training officer to rookie cops; internal MPD records obtained by investigators showed that Kueng shadowed Chauvin on 17 shifts in March 2020.

Investigators even consulted trainers from the department's academy to learn more about acceptable police procedures and twice spoke with a use-of-force expert who teaches strangulation to find out whether any of the involved officers had taken the class. They hadn't.

Both Lane and Thao were interviewed by the agency. On June 2, Thao sat down with BCA agents Reyerson and Brent Petersen and FBI agent Blake Hostetter, speaking frequently about the officers' safety, only addressing Floyd's well-being and bystanders' concerns after being prompted by one of the agents. In an un­prec­e­dent­ed move, prosecutors last August released a video of the interview.

During his interview with authorities, Lane said that while Chauvin was not his training officer, the veteran "was someone that I'd talked to for advice on calls."

Reviewing every angle

As they tried to recreate the incident, BCA agents reviewed video from every angle of the encounter, from officers' body cameras and the city surveillance camera mounted across the street from where the arrest took place to the viral Facebook Live video recorded by 17-year-old Darnella Frazier. Frazier had been on her way to Cup Foods with her cousin when she stopped to record Floyd's arrest. The footage was a key factor in the Chauvin trial, where it was played repeatedly for jurors and slowed down frame by frame.

BCA agents continued to investigate all possibilities, even though in the minds of many the video showed irrefutable evidence that Floyd suffocated under Chauvin's knee.

Using video and other sources, investigators identified at least 15 witnesses who saw what happened — including a 6-year-old boy, whose mother didn't want him to be interviewed — and another 20 people who claimed to have ridden past the scene.

Because of the weight of the video evidence, it didn't take long for prosecutors to charge Chauvin, four days after the incident. However, investigators continued to gather evidence for months leading up to Chauvin's trial.

On April 7, Reyerson took the witness stand and testified about the scope of the investigation, which involved 50 BCA agents, 25 FBI agents, 200 witnesses interviewed and 450 reports. Watching segments of the video in court, Reyerson walked the jury through when Chauvin's knee went on the back of Floyd's neck to when he fell silent and stopped moving.

Thirteen days later, the guilty verdicts were read, and Chauvin was led away in handcuffs.