Fugitive red-shirt hardliner and lese majeste suspect Wuthipong "Kotee" Kochathamakun is believed to be hiding along the Lao-Cambodian border, a senior army officer said.
Second Army chief Lt Gen Wichai Shaechoho said security agencies have closely monitored the movements of Mr Wuthipong, who reportedly fled Laos for Cambodia after Thai authorities sought cooperation from Vientiane to search for him and crack down on his group's activities.
Mr Wuthipong and his group are accused of broadcasting lese majeste content via YouTube and a local community radio station in Laos.
The Lao government agreed to cooperate, forcing Mr Wuthipong to flee, reportedly to Cambodia.
Lt Gen Wichai said it is believed Mr Wuthipong has yet to enter Cambodia and is hiding in the border area. Authorities could not give more details as to his exact whereabouts.
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Mr Wuthipong, who reportedly fled to Laos after a warrant was issued for his arrest in April 2014 on a lese majeste charge, has also been implicated in another case, which saw a cache of weapons confiscated from a house in Pathum Thani province in March.
Following the bombing at Phramongkutklao Hospital in Bangkok, army chief Chalermchai Sitthisat a few days later said new information had come to light suggesting the attack was carried out by Mr Wuthipong's network.
However, Deputy Prime Minister Prawit Wongsuwon said on Wednesday there was no evidence suggesting Watana Pumret, the arrested chief suspect in the hospital bombing, was linked to Mr Wuthipong.
Lt Gen Wichai also said the army has cooperated with neighbouring countries to closely monitor and restrict the movement of the anti-government group calling itself Sahaphantharat Thai Mai (new Thai federal state) in the Northeast as well as in some neighbouring countries.
The group was formed by lese majeste suspects and convicts including Surachai Danwattananusorn, better known as Surachai Sae Dan, and Suda Tangwatanakul.
Meanwhile, Lt Gen Wichai defended a raid conducted on Wednesday at the house of student activist Jatupat "Pai Dao Din" Boonpattararaksa in Khon Kaen province without a warrant.
The raid was aimed at maintaining law and order during a visit to the northeastern province by Prime Minister Prayut Chan-o-cha, said Lt Gen Wichai.
A total of 30 men and women police officers searched the house of Pai Dao Din's family, without a warrant. (Images FB/daodincommoner)
Citing intelligence gathered, he said there had been an attempt to discredit the government and the premier aimed at stirring up unrest, which prompted the raid.
Lt Gen Wichai insisted the military was authorised to carry out the raid to ensure peace without having to obtain a search warrant.
The military team sent out on this operation was well aware of the legal limitations of its operations in carrying out a raid without a formal search warrant, he said.
In 2014, Gen Prayut was greeted by Khon Kaen University's Dao Din student group flashing him a three-finger salute, a symbol of defiance copied from the Hollywood flick The Hunger Games.