Police were trying Monday to establish the motive behind Sunday's bomb attack in the Ramkhamhaeng area that wounded seven people.
City police chief Khamronwit Thoopkrachang said the blast could have been triggered by a business dispute or could have been an attempt to incite chaos.
However, the national police spokesman said investigators were still looking at all motives including a possible link to unrest in the far South.
"The bomb was not meant to take lives," Pol Lt Gen Khamronwit said.
"It probably wasn't linked to the southern violence."
Police have identified two possible motives, he said. "The bomb was either the result of a business conflict or an element wanting to cause trouble."
The Sunday night blast wounded seven people when it ripped through a crowded market in Bangkok's Ramkhamhaeng area near Soi Ramkhamhaeng 43/1.
Pol Lt Gen Khamronwit said police questioned City Hall officials and city inspectors about business conflicts in the area. They discovered some useful information, he said.
"We are looking to interview some people with business interests in the area," he said.
The city police chief ordered a 24-hour security beef-up at various locations across the city: Government House, parliament, the Constitution Court, the prime minister's residence, Victory Monument, Ratchaprasong intersection, department stores, the Privy Council, and the residences of the Privy Council president and senior diplomats.
Royal Thai Police spokesman Pol Maj Gen Piya Uthayo said the Metropolitan Police Bureau has been ordered to set up a panel to probe the attack.
The Hua Mak police station and Metropolitan Police Division 4 would assist with the case, he said.
Forensic evidence suggests a business conflict led to the attack, Pol Maj Gen Piya said.
However, other possible motives have not been ruled out. He said the blast could be related to southern unrest or politics.
Police had no suspects yet, he said.
A Myanmar man who was taken into custody at the scene shortly after the blast was released after being questioned.
Pol Maj Gen Piya said footage from surveillance cameras in front of Bangkok Bank and other locations in the area would provide more clues.
Police will review older cases to hunt for any connections and the case should be concluded in a week, he said.
The explosion has drawn concerns from national police chief Gen Adul Saengsingkaew.
"We are asking for seven days to solve the case," Pol Maj Gen Piya said.
The Crime Suppression Division (CSD) had combed the explosion site and suspected the bomb was likely politically motivated, a CSD source said.
The device was planted between a telephone booth and a power pylon in a manner that suggested it was aimed at causing damage rather than hurting people, the source said.
The bomb must have been planted before 6pm and set to go off two hours later, the source said.
The timing would help investigators narrow down the targets and give more clues about the motive.
He said the bomb attack appeared to be an attempt to instigate a commotion and did not seem to fit with a business conflict.
The source said the southern insurgency does not appear to be linked because the bomb was different to the devices used by militants in the far South.
Bombs used by southern insurgents are usually detonated by mobile phone and scatter steel ball bearings as shrapnel, he said. The Bangkok bomb was made with gunpowder and nails and was detonated with a timer.
Pol Col Narongrit Promsawat, superintendent of Hua Mak station, confirmed that no suspects have been detained so far.
Three of the people injured in the blast remain in hospital.
Panumet Sornsaeng, the manager of a beauty salon damaged in the explosion, Monday gave a statement to police at Hua Mak station.
He said the salon business has not led to any conflicts with anyone.
"We've been open for 20 years," he said. "The roadside stalls outside are supervised by City Hall's inspectors. We are not involved in any rentals."
Director of the National Intelligence Agency Suwapan Tanyuwattana dismissed any connection between the blast and the southern insurgency.
Government spokesman Teerat Ratanasevi said Prime Minister Yingluck Shinawatra has ordered a security beef-up.