Police said the suspected shooter, former politician Chalong Riewrang, fired at the chairman of the Provincial Administrative Organisation in Nonthaburi, which borders Bangkok, because of a disagreement over borrowed money.
"The motive was personal," Police Lieutenant General Wattana Yijin told reporters on Monday, adding the provincial official's driver was also wounded but is not in a critical state.
The official later died of his wounds at Nonthaburi's Phra Nang Klao Hospital, its director Dr Sakol Sookprome told Reuters.
In a statement to reporters while in police custody, Chalong said he had gone to confront the victim over a longstanding dispute but the latter declined to talk things out.
"But when he was driving away, he pulled a gun," Chalong said, referring to the victim: "I fired."
Police did not provide any information on whether the slain official was armed.
Thailand has been reeling from a 14-year-old boy's rampage on Friday -- first at his home and then at the Debsirin Nonthaburi School -- that left nine people dead, including the shooter, and over 20 wounded in the country's deadliest mass shooting in nearly four years.
Thai school children returned to their classrooms on Monday, some going through metal detectors and other checks, as Prime Minister Anutin Charnvirakul reiterated his intention to tighten gun controls in the wake of Friday's school shooting.
At Debsirin School in Bangkok, a sister institution to where Friday's shooting took place, authorities installed metal detectors and stepped up security as students returned to classrooms after the weekend.
School director Withan Promsintusak said officials had acted immediately after the attack, introducing weapons screening to reassure parents and protect students.
"We never imagined that a school, which should be the safest place, is where something like this would happen," he said.
Thidarat Songrat, the 54-year-old grandmother of a student, said weapons screening was necessary because schools could not otherwise determine whether students might be carrying dangerous objects.
Following Friday's shooting, Thailand's Education Ministry has said it will develop new school safety protocols over the next three months, including mental health screenings, emergency response drills, measures to tackle bullying and improved detection of weapons and other prohibited items.
Thai Premier Anutin also ordered tougher enforcement against people carrying guns in public and said police would set up checkpoints as part of a crackdown, which would include tightening controls on the possession, carrying and use of firearms.
"No one has the right to carry a firearm, especially in a public place," Anutin told reporters, adding that claiming a weapon was for self-defence was no justification.
Thailand had an estimated 10.3 million firearms in civilian possession, equating to around 15 guns per 100 residents, the highest rate in Southeast Asia by a wide margin, according to a 2017 estimate by the Small Arms Survey.
The country's current licensing regime lacks mental health screening for people applying to possess a gun, allows people as young as 20 to hold weapons and does not require periodic reassessments for licence holders, according to experts.
