The Metro Police bomb squad detonated a suspicious package - a backpack - left under a window of the Federal Courthouse at the corner of Meridian and Ohio Streets downtown.
A police K9 trained to detect explosives "hit" on the package, so technicians warned nearby hotels and detonated the backpack on the scene.
According to IMPD spokesman Michael Hewitt, the explosive material in the backpack was identified as fireworks. Another substance in the backpack was identified as marijuana.
At one point, Meridian Street between New York Street and Ohio Street was closed, but is now back open this morning.
Police say a security guard making his normal rounds located the backpack shortly before 2:40 a.m. and called in police.
Hewitt said a group of teenagers were seen in the area before the backpack turned up.
Surveillance video from cameras in the area will be used in the investigation, according to authorities at the scene.
Indiana Department of Transportation officials are warning drivers to be prepared for delays on Interstate 65 in Southern Indiana as work begins on the Ohio River Bridges Project.