The Baltimore Sun got exclusive access to the police investigation of Freddie Gray’s death, for this long piece. At the end, it recounts the shock of the investigators when Mosby charged the officers, even though their probe wasn’t yet complete:

On Friday, more remained. The task force was planning to go full­bore straight through the weekend, feeding supplemental reports to Mosby’s office, and the investigation was to remain active indefinitely, according to [task force leader Maj. Stanley] Brandford.
“Important that the state’s attorney continue to get things as we collect them,” Brandford told his tired members Friday morning.
Then his cellphone rang. He stepped into the hall and didn’t return.
Minutes later, task force members found out why: Mosby was holding a news conference on the steps of the nearby Baltimore War Memorial. From a flat­screen television in [Col. Garnell] Green’s office, they watched a live broadcast.
They stood motionless as Mosby began speaking. A lieutenant wearing a suit and bow tie rested his left hand on a leather chair; Green stood in uniform against the wall, hands behind his back. As Mosby read off the charges — including second ­degree depraved­heart murder, the most serious, against [ Officer Caesar] Goodson — stunned looks crossed their faces.
They had not expected the state’s attorney’s office to act so soon.
Later on Friday, Mosby said the charges were the result of prosecutors working 12­ and 14­hour days alongside police investigators. She also said prosecutors had been working on a “parallel investigation” that included using city sheriff’s deputies.
“This was not something that was quick, fast and in a hurry,” she said. “We reviewed hundreds of hours of camera footage and statements. This is something we worked really hard to get to the bottom of.”
Still, plenty of items remained on the police task force’s checklist. Members soon were back to their investigation.