Police have identified the suspected bomber in Sunday's grenade attack on protesters at the Victory Monument rally site, deputy national police chief Ek Angsananon said on Wednesday.
The attacker is in the same group that carried out the grenade attack on Banthat Thong Road.
The details would be revealed when sufficient evidence is secured to prove the identity of the suspect, he said.
The police now believe the attacker was in the same group that carried out Friday's grenade attack on protesters on Banthat Thong Road, said Pol Gen Ek.
He said the same group was also believed to have carried out a previous attack on Democrat Party leader Abhisit Vejjajiva's home and another attack on the home of People's Democratic Reform Committee co-leader Issara Somchai. Pol Gen Ek insisted neither the police nor the military were involved in these attacks.
Meanwhile, the city police chief has ordered a probe against Wuthipong "Ko Tee" Kotchathamakhun, a red-shirt leader in Pathum Thani, who has been accused of involvement in several attacks on anti-government protesters.
It is unclear whether or not the two developments are linked.
Metropolitan Police Bureau commissioner Kamronwit Thoopkrachang said he has instructed police to bring Mr Wuthipong in for questioning over accusations he was involved in recent violent incidents.
Pol Lt Gen Kamronwit was speaking after chairing a meeting of police teams investigating the two recent grenade attacks on Banthat Thong Road and at Victory Monument.
One protester was killed and 39 others wounded in the first attack while 28 people were injured in the second attack.
Pol Lt Gen Kamronwit said the police were instructed to find out exactly where Mr Wuthipong was at the time of the attacks.
Police were also checking whether two male suspects arrested early yesterday in Bang Khen district were connected with the bomb attacks on protesters, he said.
Krisada Saephu, 27, and an 18-year-old youth identified only as Mongkol, were detained after being found in possession of a homemade .38 pistol with 35 bullets, three other bullets of a different calibre, 23 explosive devices, a folding knife and brass knuckles.
Bang Khen police said the pair were travelling together on a motorcycle when they were stopped and searched by police on patrol near Wat Rat Niyom on Phahon Yothin Soi 52.
They told police they had been hired by an unnamed man for 5,000 baht to bring the weapons into the Saphan Mai area where someone else would pick them up.
They insisted they were not aware what weapons were inside the bags they were hired to deliver, Bang Khen police station chief Chayuth Marayat said.
Pol Col Chayuth said investigators are looking into where the weapons came from and whether the suspects and the seized weapons can be linked to a political movement.
In response to army chief Prayuth Chan-ocha's comment that he thought he already knew who was behind the Victory Monument attack, Pol Lt Gen Kamronwit said he had yet to ask Gen Prayuth about it but had confidence in military intelligence.