(Reuters) - In a gritty Boston neighborhood not long ago, where people were poor and crime was a problem, there was one three-story shingled residence widely known as "the police house," or the "House of Po'" for short.

It was also where Marilyn Mosby, Baltimore's 35-year-old African-American chief prosecutor, grew up as the daughter, granddaughter and niece of police officers.

Mosby, who took office only in January, stole the spotlight on Friday with her surprisingly swift decision to criminally charge six Baltimore police officers in the death of Freddie Gray, a 25-year-old black man.

Mosby's quick legal action stood in contrast to cases last year in Ferguson, Missouri, and New York City where, after months of investigation, prosecutors said officers had not broken the law in killings of unarmed black men.

In her announcement at a news conference in Baltimore, Mosby cited her family's "five generations" in law enforcement and lauded her "beloved" grandfather, Prescott Thompson, a charter member of the first black police organization in Massachusetts, who died in February after a long illness.

The charter was drawn up in the same house in Boston's Dorchester neighborhood.

Longtime residents remember the brown house not as a fearful presence but as a gathering place for youth. It was a place where some remember learning life's lessons.

"I spent half my life in the street and half my life at that house," said Corey Sinkler, 46. "I was a bad kid, but they saved me."

"Just because they were police, they wouldn't come out and arrest you if they saw you outside doing something wrong. They would talk to you, and set you straight," he said.

INSPIRED FROM YOUTH

Mosby's grandmother Marilyn Thompson, 77, still lives in the house with a big yard surrounded by a chain-link fence. She said her granddaughter had been inspired to get involved in law-enforcement at the age of six.

"She had fallen and hurt her leg, and we filed a claim. When she saw the judge, she said she wanted to be one. She was so impressed," Thompson said.

Mosby was also deeply influenced by a harrowing event in her youth. In 1994, her cousin, 17-year-old Diron Spence, was shot and killed just outside the family home. Family members said Diron had been riding on his bicycle when members of a street gang mistook him for someone else and opened fire.

"It was a senseless killing, and it affected all of us very much," Thompson said.

Mosby's grandfather had encouraged family, friends and neighbors to join the police, and many did.

"Back then, it was about community service. We all got involved because we were concerned about our communities," said Mosby's uncle, Harry Thompson, 56, himself a retired officer who also lives in the family home.

"These days it seems like policing is all about CYA (cover your ass)."

CHEERS AND AN OPEN LETTER

During her election campaign last year, Mosby had promised to turn back years of violent crime in the predominantly black city of 620,000 and pledged to rebuild the public's trust in the criminal justice system.

With a degree from Boston College Law School, Mosby began her legal career as a law clerk in the Baltimore City State's Attorney's Office and worked her way up as an assistant state's attorney to the general trial division. During her campaign, she said she had an overall 80 percent conviction rate.

Mosby left to work as an insurance company lawyer before seeking the state's attorney's job.

In the Democratic Party primary in June, Mosby defeated the incumbent with 54 percent of the vote. She went on to trounce her opponent in the November election.

"Police brutality is completely inexcusable. I'm going to apply justice fairly, even to those who wear a badge," Mosby told the Baltimore Sun newspaper last year.

Mosby drew cheers from a crowd at the Baltimore news conference where she announced the charges on Friday in the Gray case. She said Gray had suffered a critical neck injury while riding in a police van.

Prosecutors charged the driver of the van with second-degree murder, alleging callous indifference. Gray died later in hospital.

The city's Fraternal Order of Police Lodge No. 3 defended the officers and said Mosby's marriage to Baltimore City Councilman Nick Mosby posed a conflict of interest.

At the news conference, Mosby asserted the independence of her office's investigation.

"I thought it was very important to have an independent analysis as to what took place and transpired from the very beginning. We are an independent agency from the police department," Mosby said.