There is no clear indication of when Kehoe conceived the idea of massacring the schoolchildren and townspeople, but Ellsworth, who was a neighbor, thought that Kehoe conceived his plan after being defeated in the April 5, 1926, township clerk election.
The general consensus of the townspeople was that he had worked on his plan at least since the previous August.
Bath School Board member M. W. Keyes was quoted by The New York Times:
I have no doubt that he made his plans last Fall [1926] to blow up the school ... He was an experienced electrician and the board employed him in November to make some repairs on the school lighting system.
He had ample opportunity then to plant the explosives and lay the wires for touching it off.

Kehoe had free access to the school building during the summer vacation of 1926.
From mid-1926, he began buying more than a ton of pyrotol, an incendiary explosive used by farmers during the era for excavation and burning debris.
In November 1926, he drove to Lansing and bought two boxes of dynamite at a sporting goods store.
Dynamite was also commonly used on farms, so his purchase of small amounts of explosives at different stores and on different dates did not raise any suspicions.
Neighbors reported hearing explosions set off on the farm, with one calling him "the dynamite farmer".
Following the disaster it was reported that Michigan State Police investigators had discovered that a considerable amount of dynamite had been stolen from a bridge construction site and that Andrew Kehoe was suspected of the theft.
Investigators also recovered a container of gasoline in the school's basement.
The container was rigged with a tube and investigators speculated that Kehoe had planned that the gasoline fumes would ignite from a spark scattering burning gasoline throughout the basement.
In the undamaged section of the school it was found that Kehoe had concealed the explosives in six lengths of eavestrough pipe, three bamboo fishing rods and what were described as "windmill rods" that were placed in the basement ceiling.

Kehoe purchased a .30-caliber Winchester bolt-action rifle in December 1926, according to the testimony of Lieutenant Lyle Morse, a Michigan State Police investigator with the Department of Public Safety.
There is no clear indication of when Kehoe conceived the idea of massacring the schoolchildren and townspeople, but Ellsworth, who was a neighbor, thought that Kehoe conceived his plan after being defeated in the April 5, 1926, township clerk election. The general consensus of the townspeople was that he had worked on his plan at least since the previous August. Bath School Board member M. W. Keyes was quoted by The New York Times:
I have no doubt that he made his plans last Fall [1926] to blow up the school ... He was an experienced electrician and the board employed him in November to make some repairs on the school lighting system. He had ample opportunity then to plant the explosives and lay the wires for touching it off.
Kehoe had free access to the school building during the summer vacation of 1926. From mid-1926, he began buying more than a ton of pyrotol, an incendiary explosive used by farmers during the era for excavation and burning debris. In November 1926, he drove to Lansing and bought two boxes of dynamite at a sporting goods store. Dynamite was also commonly used on farms, so his purchase of small amounts of explosives at different stores and on different dates did not raise any suspicions. Neighbors reported hearing explosions set off on the farm, with one calling him "the dynamite farmer". Following the disaster it was reported that Michigan State Police investigators had discovered that a considerable amount of dynamite had been stolen from a bridge construction site and that Andrew Kehoe was suspected of the theft. Investigators also recovered a container of gasoline in the school's basement. The container was rigged with a tube and investigators speculated that Kehoe had planned that the gasoline fumes would ignite from a spark scattering burning gasoline throughout the basement. In the undamaged section of the school it was found that Kehoe had concealed the explosives in six lengths of eavestrough pipe, three bamboo fishing rods and what were described as "windmill rods" that were placed in the basement ceiling.
Kehoe purchased a .30-caliber Winchester bolt-action rifle in December 1926, according to the testimony of Lieutenant Lyle Morse, a Michigan State Police investigator with the Department of Public Safety.
Bath School disaster