Thai police said on Saturday they were not ruling out either political or "personal" motives in an assassination attempt on the founder of the "yellow shirt" protest movement.
Sondhi Limthongkul survived after gunmen riddled his car with bullets at a petrol station before dawn on Friday. Doctors said his life was not in danger.
His People's Alliance for Democracy played no part in the political violence in Thailand over the past week.
However, it was involved in a street campaign led to the military coup that ousted Prime Minister Thaksin Shinawatra in 2006. A similar campaign helped undermine two pro-Thaksin governments in 2008.
Deputy Police Chief General Jongrak Juthanont said evidence suggested five people were involved in the assassination attempt.
A total of 84 spent cartridges were found at the site of the shooting, near the central bank in Bangkok.
Mr Sondhi's driver is still in critical condition and doctors plan to do a third operation on him. A bodyguard suffered minor wounds.
The assassination attempt on Friday came several hours before the government extended a state of emergency in the capital to help it restore order after a week's violence involving rival "red shirts" loyal to Mr Thaksin.
A summit of Asian leaders in Thailand had to be cancelled last weekend after protestors invaded the venue in Pattaya.
Protests were than staged in Bangkok, in which two people died.