The Democrat Party has urged police to speedily find the gunman who fired bullets into four ASTV Manager cars at the newspaper's head office in the early hours of Saturday.
Opposition leader Abhisit Vejjajiva said the incident represented a threat to the media, and police had a responsibility to quickly shed light on who was the shooter.
Police are searching for clues to the identity of the gunman.
ASTV, which backs the yellow-shirt People's Alliance for Democracy, was recently involved in a war of words with the army chief, Gen Prayuth Chan-ocha. Soldiers angered by its criticism of their boss staged two rallies at the ASTV offices on Jan 11 and 12 before being ordered to stop.
An officer is reflected in the shot-up side window of one of the four ASTV cars hit by gunfire early Saturday morning. (Photo by Chanat Katanyu)
Pol Lt Gen Kamronwit Thoopkrachang, the Metropolitan Police Bureau chief, called for patience as officers were trying to solve the case, instead of pointing fingers at the army.
Any number of suspects were possible, including soldiers or those trying to stir up trouble in the country, he said.
The answers, he added, would depend on the evidence collected from the crime scene by investigators.
The army has not reacted to the Saturday shooting.
Pol Lt Gen Kamronwit said earlier that police would have to discuss possible motives with ASTV management.
The ASTV Manager newspaper is a vocal critic of the government and Prime Minister Yingluck Shinawatra's elder brother, Thaksin.
Police said someone with a handgun fired at the cars, which were parked in front of the ASTV headquarters at Baan Chao Phraya and nearby Santi Chaiprakan Park on Phra Arthit Road.
Pol Lt Nopasit Piyarath, an investigator at the Chana Songkhram station, said he received a report on the attack about 3.25am. The shooting caused damage to the four vehicles but no one was injured.
ASTV security guard Rungrote Khaoprasert, 34, told police that the four news cars were parked on the sidewalk because the car park inside the ASTV headquarters compound was full.
While walking to the restroom at about 3.25am, he heard four shots but did not think they were from nearby. When reporters came to take the vehicles to go to work in the morning, they found the four cars damaged by bullets, Mr Rungrote said.
The bullets were believed to have been fired from a .22-calibre handgun.
Police recovered closed-circuit TV security footage and noted a suspicious man in a black shirt walking on the footpath opposite Baan Chao Phraya at 3.25am.
At 3.26 the footage showed a picture of one damaged car, and at 3.28, it showed the same man walking back, heading to nearby Soi Kai Jae, which connects to several nearby roads.
Police also found a mirror on the second floor of the Anurak building, which is the management office of ASTV, was broken by the shooting.
Police would also monitor other footage from nearby CCTV cameras to hunt down the attackers, said Pol Lt Gen Kamronwit.
The Thai Journalists Association issued a statement condemning the attack. It called on all parties to respect media freedom and refrain from using violent means to threaten journalists performing their duty for the public.
It said that if people felt they were affected by biased reporting, they could file complaints with the National Press Council or pursue more serious cases in the courts.
The statement also called on the media to perform their duty based on professional ethics and without any bias that could lead to a distortion of facts, and refrain from using rude or insulting language.
ASTV-Manager was founded by Sondhi Limthongkul, one of the core leaders of the PAD.
The PAD of late has been rallying "patriots" who feel the government is prepared to concede defeat in the decades-old border dispute with Cambodia over the Preah Vihear Temple.
ASTV Manager recently targeted Gen Prayuth in a commentary that said he had failed to stand up for the country's interests over Preah Vihear. It also criticised his handling of army strategy in the troubled southern provinces.
Gen Prayuth responded by calling ASTV Manager a "lousy newspaper". The paper then said he was behaving "like a woman on her period". Soldiers rallied in front of the newspaper's offices on Jan 11 and 12 before Gen Prayuth ordered them to stand down.
Sondhi became a vocal supporter of Thaksin Shinawatra after state-owned Krung Thai Bank, during Thaksin's premiership, forgave one billion baht in debts owned by Manager Group.
Viroj Nualkhair, a former financial adviser to Sondhi, was the president of KTB when the loans were forgiven. Mr Viroj was later forced out of the bank amid investigations into irregular loan deals worth up to 40 billion baht. Criminal proceedings are still under way in some cases.
Sondhi and Thaksin subsequently had a falling out, for reasons that are not clear. After that, Mr Sondhi stepped up his attacks on the Thaksin government in his media outlets.
Sondhi was seriously wounded in an assassination attempt in 2009 when gunmen fired more than 100 M-16 and AK-47 assault rifle rounds at his car. Police and soldiers both came under suspicion but the assailants have never been found.
Sondhi was convicted in February last year after pleading guilty to violating the Securities and Exchange Act by falsifying documents to secure a loan of 1.1 billion baht for his media empire in the 1990s.
He faces up to 20 years in prison but remains free on bail, and still travels abroad, pending an appeal against the sentence.