Speedway authorities called on explosive experts from the Indiana State Police, the Marion County Sheriff's Department, the 64th Ordnance from nearby Fort Benjamin Harrison and specialists from the Bureau of Alcohol Tobacco and Firearms (ATF).

[1] The last bombing took place on September 6, when another explosive device concealed in a gym bag detonated in the parking lot of Speedway High School shortly after a freshman football game.

[3][4] As no motive ever emerged for the bombings, law enforcement had no idea why they had stopped, but on September 20, 1978, federal agents arrested 27-year-old Brett Kimberlin for attempting to obtain United States Government credentials illegally.

[3] After obtaining a search warrant for Kimberlin's home and vehicle, investigators found wiring similar to those used on the explosive devices and "Mark Time" appliance timers in his 1970 Chevrolet Impala.

A subsequent search of his home revealed more than 1,000 pounds (450 kg) of marijuana and two cases of Tovex 200, used in the IEDs, whose purchase had been traced by their lot number to Kimberlin in 1975.

[1] The ATF positively matched both the timers and wire found in Kimberlin's car but did not press for an immediate indictment; instead, they continued to build on their case.

[3] While no motive was established at trial, prosecutors and police believe Kimberlin went on the bombing spree to deflect attention away from an ongoing investigation of the murder of 65-year-old Julia Scyphers.

[11] His parole was revoked and he was returned to prison in 1997 after not making court ordered payments to the DeLong family which resulted from their successful civil suit.