Thursday’s announcement was months in the making. Last fall, federal prosecutors spent weeks interviewing more than a dozen victims and their attorneys about their experiences with the department. Michael Osgood, who has led the Special Victims Directorate for eight years, said the federal investigation is an indictment of Mayor Bill de Blasio and senior police chiefs working under him. Last year, Mr. Osgood, who resigned in 2018, sued the administration. he and many lawyers believe he was forced to leave because he cooperated with the Ministry of Research. The report was based on dozens of memos from senior officials. The mayor and his subordinates “for years were fully aware of the root causes of the Special Victims system failures and did very little to correct them, and in fact made it worse,” Osgood said. “They were dishonored and ashamed in a respectable department. “Now there is hope.” Mr de Blasio did not immediately respond to a call for comment. Mayor Eric Adams told a news conference Thursday that he welcomed the latest review. “There is no higher level of priority for us than ensuring that victims of sexual assault receive the right treatment, research and solution,” he said. While many welcomed the federal investigation in New York, some victims felt conflicted. Alison Turkos had told police she was abducted, held at gunpoint and raped in October 2017 by a Lyft driver and two other men. Federal investigators took over the case after evidence showed she had been abducted by state lines. The announcement came less than a day after he learned he would not be charged. “There are so many of us, including myself, that we will not even get the impression of justice or accountability because our legal system failed miserably, because we did what we had to do,” he said, describing how eager he was to help the researchers. “It feels like a slap in the face.” Glenn Thrush, Emma Fitzsimmons and Benjamin Weiser contributed to the report.