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New Freddie Gray video footage released

Gray died Sunday of what his family's attorney said was a severed spine that allegedly occurred after he was arrested on a weapons charge in Baltimore.

Anticipating the city's biggest protest yet in the aftermath of the unexplained police-custody death of Freddie Gray, the Baltimore Police Department on Friday offered an update on the investigation and tweeted new video surveillance footage of Gray from the day of his arrest.

Gray, a 25-year-old black man, died Sunday of what his family's attorney said was a severed spine that allegedly occurred after he was arrested on a weapons charge in Baltimore on April 12. 

At a press conference Friday, Police Commissioner Anthony Batts said investigators were narrowing down the timeline of events that led to Gray's death, but that many details remain unclear. However, Batts did say that Gray was not buckled down in the transportation wagon that carried him to the police station -- something for which the commissioner said there is "no excuse." Baltimore Police Department rules, updated nine days before Gray's arrest, state that detainees must be seat belted during transportation.

The Baltimore Police Department has received a preliminary, verbal report from the medical examiner, Batts said, but that a toxicology report would take an estimated 30-45 days to complete.

RELATED: DOJ opens probe of police custody death of Freddie Gray

Responding to calls for his resignation, Batts said, "that's not gonna happen," touting his own record as a "reform commissioner," and asking the public to refocus its attention instead on the fact that "a young man has lost his life."

Deputy Commissioner Kevin Davis offered a chronology of events that police believe took place the day of Gray's arrest. Three bike cops spotted Gray walking down the sidewalk, Davis said, when Gray and an as-yet-unidentified person fled. Gray was apprehended and that is when he "should have received medical attention and did not," he added.

Gray was then placed in a transportation wagon, the deputy commissioner continued. The vehicle stopped, leg irons were attached to Gray, and he was placed back into the wagon. The vehicle stopped for a second time, and the facts of that interaction, Davis said, remain under investigation. At a third stop, a separate arrest scenario occurred that Davis said required additional officers to respond. A second prisoner was brought into the vehicle, which then carried Gray to the Western District, at which point an ambulance was called and he was transported via ambulance to "shock trauma" where he died a week later.

The officers, who can be seen on cellphone video pinning Gray to the ground before loading him into the van, have been suspended with pay while the police department and federal authorities conduct investigations.

RELATED: Six cops suspended in death of Freddie Gray

Earlier Friday, Baltimore Mayor Stephanie Rawlings-Blake demanded answers to questions surrounding Gray's death. "This is absolutely unacceptable, and I want answers," Rawlings-Blake said after meeting with protesters calling for justice. The mayor said she wanted to know why none of the officers immediately called for medical assistance and why "policies for transport" were not followed.

After a week of protests, activists promised their biggest march yet Saturday, saying they would try to "shut down" the city. Deputy Commissioner Paul Meer said, "We expect traffic delays through the city."

A wake for Gray will be held Sunday. The funeral is set for 11 a.m. Monday at New Shiloh Baptist Church.

Additional reporting by NBCNews' Elisha Fieldstadt.