Mosby said her office has also found probable cause to pursue criminal charges in connection to the case. Six officers involved in putting Gray in custody and transporting him in the police wagon April 12 are facing charges.
Here is what to know about the woman at the center of today's big announcement:
She's the youngest chief prosecutor in a major city:
Mosby, a mother of two and West Baltimore resident, started her job as State Attorney in January. She is 35 years old, making her the youngest chief prosecutor of any major city, the State Attorney website says.
Today, she said the youth of Baltimore is at "the forefront" of the Gray cause.
"To the youth of this city: I will seek justice on your behalf. This is a moment. This is your moment," she said. "As young people, our time is now."
She comes from a family of police officers:
Mosby isn't new to working with law enforcement. She noted she comes from five generations of police officers.
"My father was an officer, my mother was an officer, several of my aunts and uncles," she said at today's news conference. "My recently departed and beloved grandfather was one of the founding members of the first black police organization in Massachusetts."
Mosby added that today's charges shouldn't reflect on the work of the Baltimore Police Department as a whole, or on the cooperation between her office and the force.
Her husband is a Baltimore City Councilman:
She has strong ties to the Baltimore community -- her husband is Baltimore City Councilman Nick Mosby, according to the State's Attorney website, and she has a "personal and professional relationship" with Gray's family's attorney, according to the Baltimore City Fraternal Order of Police.
An open letter sent by the Fraternal Order of Police before Mosby's news conference today requested that Mosby appoint a Special Independent Prosecutor to "avoid any appearance of impropriety."
"I have full faith in your professional integrity," the letter said. "While I have the utmost respect for you and your office, I have very deep concerns about the many conflicts of interest presented by your office conducting an investigation in this case. These conflicts include your personal and professional relationship with Gray family attorney, William Murphy and the lead prosecutor’s connections with members of the local media. Based on several nationally televised interview, these reporters are likely to be witnesses in any potential litigation regarding this incident. Most importantly, it is clear that your husband’s political future will be directly impacted, for better or worse, by the outcome of your investigation."