Police say they are still in the dark over who attacked anti-government People's Democratic Reform Committee (PDRC) protesters on Ratchadamnoen Avenue early Thursday.
Three people were killed and 22 wounded in the drive-by shooting and grenade attacks early Thursday.
Metropolitan Police Bureau chief Kamronwit Thoopkrachang said investigators were struggling to identify potential suspects because of a lack of evidence.
Pol Lt Gen Kamronwit said the only clues police had were images of a suspicious looking vehicle seen heading towards the scene of one of the attacks.
The shooting occurred at 2.45am, when men in a white pickup truck drove by and shot at a line of PDRC guards at Khok Wua intersection.
Two people were killed.
They were identified as Narayot Chanphet, 21, a PDRC guard who was shot in the chest, and Somkhuan Nuankanai, 51, a protester.
Five minutes later, attackers fired two M79 grenades at the anti-government protesters camping out at the Democracy Monument.
The army arrived soon after the attacks, sealing off the area to allow investigators to gain access.
A third man later died from wounds suffered in the attack near the Democracy Monument.
The victim, who wasn't named but was a member of the protest gathering, died at Hua Chiew Hospital.
The violence took the death toll in the PDRC's six-month anti-government campaign to 27, the Erawan Emergency Centre said.