Transport Minister Saksayam Chidchob suspects human error caused the fatal bridge collapse in Samut Sakhon on Sunday night.
The U-turn bridge collapse onto the Rama II Highway in Muang district killed two people and injured two others.
The minister was answering reporters' questions at Government House on Tuesday.
Mr Saksayam said the fall of a huge concrete beam from the flyover while it was undergoing repair could be the result of human error, but he would rather wait for the result of the official investigation by experts. He expected the findings within 14 days.
Minister Saksayam said he had earlier ordered the Highways Department to suspend any highway maintenance and construction during times of high traffic on long holidays.
The incident happened on the Bangkok-bound side of Rama II Highway at the end of last week's long weekend, which started on Thursday. Rama II is the main highway linking Bangkok and the South.
On Monday Highways Department director-general Sarawut Songwilai said the bridge collapse occurred while workers were about to pour the new concrete surface of the bridge.
The old surface had been gouged out. The outer beam of the bridge fell without warning onto the Rama II Highway, he said.
The highways chief said his department took full responsibility because its bridge construction and maintenance centre based in Pathum Thani province took up the bridge repair and did not hire a contractor to do it.
He was also waiting for the investigation finding. If any official was found responsible, the department would impose strong punishment, Mr Sarawut said.
On Sunday night a five-tonne concrete beam fell from the U-turn bridge directly onto a car passing below. Front seat passenger Suwannee Rakthuam, 41, from Bangkok's Chomthong district, was killed instantly. The car driver, driver Lakhana Jongsiriroj, was injured and taken to hospital.
Part of the beam also hit the front of a pickup truck and an unloaded diesel tanker.
Two workers fell from the bridge with the beam - Chan Chaothong, 48, of the Highways Department's bridge construction and maintenance centre in Pathum Thani, and Chatchai Sirimat, a casual employee. Chan died from his injuries.
The inbound express lanes of the national highway remained closed while the structural safety of the overbridge was checked.
Meanwhile, Thanes Weerasiri, president of the Engineering Institute of Thailand, said that if the bridge surface was taken off before removing its guard rail, the outer beam with the guard rail could fall like the incident on Sunday, as the bridge surface was designed to rest on the beam and secure it.
For the damaged U-turn bridge, the guard rail must be removed first, he said.
The Highways Department later stated that its officials would remove all the remaining beams of the bridge to allow the reopening of express lanes on Rama II Road at noon on Wednesday.