The Aug. 4 attack, which ended when
police shot and killed the gunman, was one of a series of three-high profile mass shootings that stunned the
United States and stoked its long-running debate on gun rights.
The office of U.S. Attorney Benjamin Glassman said charges against the friend, who was not identified, would be unsealed at 2 p.m. (1800 GMT) in Dayton.
The
FBI said last week that the slain suspect, 24-year-old Connor Betts, had a history of violent obsessions and had mused about committing mass murder before his rampage in Dayton's historic downtown.