President Barack Obama's nominee for US Attorney General launched a civil rights investigation into the choking death of Eric Garner. View Full Caption DNAinfo/Theodore Parisienne

BROOKLYN — Federal investigators launched a civil rights investigation into the death of Eric Garner, Mayor Bill de Blasio and Attorney General Eric Holder said Wednesday, the day a grand jury cleared an NYPD officer in the Staten Island man's death.

"Our prosecutors will conduct an independent, thorough, fair and expeditious investigation," the attorney general said in a televised press conference.

De Blasio told reporters at a Staten Island press conference that Attorney General Eric Holder and Brooklyn U.S. Attorney Loretta Lynch, who was nominated for the country's top law enforcement spot in early November, had assured him that they were looking into the matter.

"They made clear that the investigation initiated by the U.S. Attorney will now move forward," the mayor said. "[They said] that it will be done expeditiously. It would be done with a clear sense of independence and that it will be a thorough investigation.”

Garner's mother, Gwen Carr, said she was optimistic about the AG's involvement.

“We are so happy that the federal government is taking the case,” she said.

But legal experts said they were sceptical that the facts of this case could lead to a prosecution.

In August, Holder launched a civil rights investigation into the death of Michael Brown, the unarmed 19-year-old shot dead by a police officer in Ferguson, Mo. Last week, Darren Wilson, the officer responsible for the shooting was cleared by a Missouri grand jury.

“I am confident that Loretta Lynch will conduct a tough but fair investigation into the tragic death of Eric Garner," U.S. Sen, Charles Schumer said. "She is both a consummate professional with a strong commitment to justice, and has a close working relationship with the New York City law enforcement community.”

While she was an assistant U.S. attorney in 1997, Lynch oversaw the prosecution of police officers who brutalized Abner Louima in a Brooklyn precinct bathroom.

More recently, she declined to bring civil rights charges against several officers who shot and killed Sean Bell, an unarmed man celebrating his bachelor party.

"Mr. Garner’s death is one of several recent incidents across the country that have tested the sense of trust that must exist between law enforcement and the communities they are charged to serve and protect," said Holder. "We must seek to heal the breakdown in trust that we have seen."