CHICAGO | Joe Jackson was a 59-year-old man with a quiet, unremarkable life who was convinced his situation would be transformed by his invention, a specially designed toilet for truck drivers.

Family and neighbors were bewildered Sunday how accounts of the gun-toting, ranting killer could match the person they knew — a father and churchgoer who lived on the city’s West Side.

“I believe he just snapped,” son Darrin Jackson, 39, told the Chicago Sun-Times. “I believe he was just frustrated.”

At the end of the bloody, 45-minute attack, three employees of a downtown law firm — including one attorney whose business card Jackson kept in his pocket — were dead, shot in the head and neck, and another was wounded.

Jackson did not stand out as a dangerous character, and was involved at his church, neighbors said.

Jackson forced a security guard at gunpoint to the 38th floor offices of Wood, Phillips, Katz, Clark & Mortimer, police said. Once inside, he locked and chained the door and demanded to see lawyer Michael R. McKenna, a patent attorney who rented office space from the firm.

Jackson went to the firm looking for McKenna, but investigators were still trying to decipher if the men were connected, including whether Jackson had been a client, said Police Superintendent Phil Cline.

Jackson was convinced that attorneys were trying to steal his invention, his family said.

Niece Brenda Jackson said someone at McKenna’s law office hung up on a phone call from him. He tried to enter the office at least one other time Friday, but was turned away because he did not have an appointment, police said.

“He went to them, he trusted them,” Brenda Jackson told the Sun-Times. “He said, ‘This is gonna pay off big.'”

The Associated Press left messages at numerous phone listings for Brenda Jackson in the Chicago area. A telephone listing for Darrin Jackson was unlisted.

McKenna, 58, and Paul Goodson, 78, a retired teacher who worked part time in the office, both died from a gunshot wound to the head, the Cook County medical examiner’s office said Sunday.

Allen J. Hoover, 65, a partner at the intellectual property and patent firm, died from a gunshot wound to the neck, the office said.

Jackson shot McKenna’s paralegal, Ruth Zak Leib, in the foot, police said. Her husband, Larry Leib, told the Chicago Tribune that Jackson ordered his wife at gunpoint to identify McKenna.

SWAT snipers killed Jackson — who was also armed with a knife and a hammer — shooting him in the face and chest after he grabbed a hostage, police said.