Baltimore's top prosecutor on Wednesday dropped remaining charges against police officers tied to the death of black detainee Freddie Gray, after failing four times to secure convictions in a case that inflamed the U.S. debate on race and justice, Reuters reported. Prosecutor Marilyn Mosby had stunned the city and became a national figure by filing charges against six officers just days after Gray's death from a broken neck suffered in a police van sparked protests and rioting in April 2015. The death of the 25-year-old was among high-profile deaths of black suspects at the hands of U.S. police that stoked a national debate on police tactics and the treatment of minorities. It fueled the rise of the Black Lives Matter movement, which protests excessive police use of force against minorities. The decision to drop charges against the three remaining officers facing trial came on the day of a pretrial hearing for Officer Garrett Miller. His trial was to start on Thursday in Baltimore City Circuit Court. At a news conference held before a mural in Gray's neighborhood memorializing him, a combative Mosby said that individual police officers had tried to thwart her investigation. The interference included officers who were witnesses investigating the case and key questions not being asked during interrogations. A police counter-investigation aimed at disproving the prosecution's case also failed to execute search warrants, Mosby said. "Police investigating police, whether they are friends or merely their colleagues, was problematic," she said to cries of "we're with you" from onlookers. Successful prosecution was impossible without an independent investigation, a say in whether the cases would be heard before a judge or jury, community oversight of police and major justice reforms, she said. A Baltimore police spokeswoman had no immediate comment. The officers still face administrative reviews over Gray's death.