US Transport Secretary Pete Buttigieg toured the wreckage of this month's train derailment and toxic spill in East Palestine, Ohio, on Thursday while federal investigators released their initial report on the February 3 incident.
Little new information emerged from the National Transportation Safety Board's preliminary report about the derailment of the Norfolk Southern train loaded with toxic chemicals.
The agency said the investigation was focusing on the possibility that an overheated wheelset and bearing on a car triggered the derailment.
Wearing a hard-hat and orange safety vest, Mr Buttigieg met NTSB staff at the scene of the derailment, which sparked the evacuation of thousands of people and ignited health concerns.
The administration of President Joe Biden had been criticised for not having made a high-level visit sooner. Environmental Protection Agency administrator Michael Regan visited East Palestine earlier this week.
The NTSB was due to hold a news conference to further detail its initial findings.
It also said it was investigating whether pressure relief valves on train cars carrying the toxic chemical vinyl chloride functioned properly following the wreck and subsequent fire.
Experts had said that if those relief valves had functioned properly, authorities may not have needed to drain upwards of 1.1 million pounds of vinyl chloride from cars and purposefully set it ablaze, releasing heavy contamination into the environment.
The NTSB said investigators were also looking at how a defect detection system worked and whether the system might have alerted the train engineer to the problems earlier.
Norfolk Southern's chief executive apologised on Wednesday at a CNN town hall event that highlighted residents' concerns about soil and groundwater contamination.
Mr Biden, a Democrat, and his administration have said the company must pay for the damage and clean-up efforts, and the EPA ordered company officials to attend town hall events after executives initially boycotted.
Mr Buttigieg had said he would push major railroads to improve safety and seek bipartisan support in Congress to raise the cap on fines against railroads for breaching safety regulations.
Some Republicans have criticised the Biden administration over the incident while some Democrats have pointed to regulations rescinded under former president Donald Trump, a Republican.
Mr Trump, who is campaigning for the 2024 Republican presidential nomination, visited the area on Wednesday.
