The Criminal Court in Bangkok on Thursday acquitted red-shirt supporter Amporn Jaikorn, 56, of involvement in an explosion at an apartment building which killed four people and injured nine others in Nonthaburi province in 2010.
The court ruled that the prosecution had presented only two police officers as witnesses. One of them only received reports on the explosion, and the other inspected the bomb scene afterwards. There was no eyewitness, and Ms Amporn had denied any involvement.
Prosecutors described Ms Amporn, a native of Chiang Mai province, as a member of the red-shirt United Front for Democracy against Dictatorship (UDD).
She had been charged with possessing five fire extinguisher and refrigerant cylinders containing 20 kilogrammes of explosive, and an AK-47 assault rifle and 129 rounds of ammunition, at Saman Metta Mansion apartment building in Bang Bua Thong district, Nonthaburi.
The bomb went off about 6pm on Oct 5, 2010, and ripped through the first and second floors of the apartment block.
Ms Amporn said she was happy with the judgement. Her lawyer Benjarat Methian said she would cite the acquittal in support of an application to Min Buri Court for the temporary release of her client on bail.
Ms Amporn is being held in Min Buri prison pending prosecution in relation to a motorcycle explosion which killed two people in Min Buri district on March 29, 2014.
Authorities believed one of the four people killed in the explosion at the five-storey Saman Metta Mansion was a man who was making bombs in a room he rented in the building. They quoted Ms Amporn as admitting she knew the man, identified as Samai Wongsuwan of Chiang Mai's Hang Dong district.
The explosion on the evening of Oct 5, 2010, occurred just hours after then-prime minister Abhisit Vejjajiva's government announced that Bangkok and three surrounding provinces would remain under emergency rule amid the continued activities of the UDD.
That followed the violent UDD-led mass demonstrations against the government earlier in the year and the use of the military to disperse the rallies.