(CNN) Lowndes County, Alabama, Sheriff "Big John" Williams was shot to death Saturday at a gas station where people often hung out -- including Williams' own son, the sheriff from a neighboring county told CNN on Monday.

"The devastating part... his son was at the gas station that same night. He witnessed all of this," Montgomery County Alabama Sheriff Derrick Cunningham told CNN's Brooke Baldwin.

In a separate phone interview, Cunningham confirmed the victim's son was in the store shopping when his father was shot.

The 18-year-old suspect, William Chase Johnson, is the son of a Montgomery County deputy, Cunningham said.

William Chase Johnson is the son of a Montgomery County sheriff's deputy.

Johnson was arrested around midnight Saturday after returning to the same gas station where Sheriff Williams was shot, Sgt. Steve Jarrett with the Alabama Law Enforcement Agency said. A handgun was taken from him, Jarrett said.

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