The U.S. Justice Department said Tuesday it has launched a civil-rights investigation into the death of a 25-year-old black man who suffered spine injuries and lapsed into a coma after his arrest by Baltimore police.

“Based on preliminary information, the Justice Department has officially opened this matter and is gathering information to determine whether any prosecutable civil-rights violation occurred,” spokeswoman Dena Iverson said. The announcement came after five members of the state’s congressional delegation asked...