Former MP Chalong Riewraeng cited a debt dispute as prompting him to fatally shoot Nonthaburi Provincial Administration Organisation (PAO) chief Thongchai Yenprasert on Monday, a senior officer said.

Pol Lt Gen Wattana Yeejeen, commander of Provincial Police Region 1, said Mr Chalong claimed Thongchai had refused to repay an outstanding debt.

The officer did not specify the amount, but earlier reports said the sum was 11 million baht.

Mr Chalong made the claim during interrogation after he was arrested at Thongchai’s office and taken to Rattanathibet police station for questioning.

Thongchai, 71, a former police officer with the rank of police colonel, was pronounced dead at Phranangklao Hospital about four and a half hours after the shooting, which occurred at about 11am in the PAO office in Nonthaburi's Muang district.

The PAO office is located in a complex of provincial state offices which also comprises the provincial hall and other agencies.

Preliminary findings indicate the former lawmaker waited outside the PAO chief’s office on the third floor of the building. Thongchai refused to meet him and hurried to his Volvo van parked behind the premises.

Thongchai was seated beside his driver, known only as Yod, when Mr Chalong followed him downstairs and signalled for the PAO chief to lower the front window. The former MP then produced a Beretta pistol, fired four shots at Thongchai and shot at Mr Yod as he ran away. Mr Chalong remained at the scene and waited to be arrested.

Thongchai received cardiopulmonary resuscitation and was taken to hospital before he succumbed. Mr Yod suffered minor injuries and is receiving treatment.

"I never imagined I'd be in this position (speaking to police and reporters at a police station)," the former politician said. "I came to speak to a friend, but he refused to listen."

"I told Daeng (Thongchai's nickname) to lower the window, but he would not. I then drew a gun and demanded to talk, but Yod also produced a pistol, so I opened fire."

Pol Lt Gen Wattana said police have not yet pressed charges and have not ruled out other possible motives.

Investigators are compiling evidence to determine whether the killing was premeditated or the result of a spontaneous act, he added.