Investigators are searching for clues as to why a car that was being driven towards the Canadian border from the US accelerated before crashing and bursting into flames.

The Wednesday morning incident happened on the Rainbow Bridge at the US-Canada border near Niagara Falls.

The couple in the car were killed, and a US border agent was injured.

The bridge is closed for a second day as police work to determine what happened.

The crash initially triggered a major security scare on the eve of Thanksgiving, with authorities closing several ports of entry between Canada and the US in the area as a precaution.

New York Governor Kathy Hochul later ruled out terrorism, but many unknowns remain, including the identity of the couple in the car.

The explosion happened at around 11:30 local time (16:30 GMT) on Wednesday, causing serious disruption on one of the busiest travel days of the year ahead of American Thanksgiving.

Gov Hochul said the car travelled at a “very high rate of speed”, hurtling over an 8ft (2.4m) fence on the New York side of the border near a checkpoint.

The vehicle was “incinerated”, Ms Hochul said, and nothing was left but the engine. Not even a registration plate was recovered.

US law enforcement confirmed the two people inside the car were a married couple. But it remains unclear why they crashed.

Robert Restaino, the mayor of Niagara Falls in New York, told the New York Times that the couple were business owners from nearby Grand Island and were both in their 50s.

Mr Restaino said they were driving an older Bentley model and were on their way to a concert in Canada, believed to be a Kiss show that was cancelled.

Among other possibilities, Mr Restaino said police were looking at whether the car had suffered a mechanical failure that caused it to accelerate.

A casino located less than a mile from the Rainbow Bridge on the New York side of the border, the Seneca Niagara Resort and Casino, where the vehicle stopped briefly, has said it is cooperating with the ongoing police investigation.

The Rainbow Bridge is a crossing for motor traffic and pedestrians over the Niagara River, and connects the cities of Niagara Falls, Canada, and Niagara Falls, in New York state.

The mayor of Niagara Falls, Canada, Jim Diodati, told Canada’s public broadcaster he expected the crossing to reopen some time on Thursday or Friday.

“Today is a holiday in the US, so that may delay things,” he said.

Mr Diodati described the immediate aftermath of the blast as an “emotional roller-coaster” as officials struggled to sift through what had happened, and there were initial fears of a terrorist attack.

The Federal Bureau of Investigations of Buffalo said in a statement that it had concluded its investigation at the scene.

It found “no explosive materials, and no terrorism nexus was identified”.

The matter has been handed over to the Niagara Falls Police Department as a traffic investigation. The force said it was working to identify those involved and notify their next of kin, but that due “to the complexity of the incident, the investigation will take some time”.

The closure has affected both Canadians and Americans in the area. One eyewitness, Toronto resident Dor Tamang, said he was processing immigration documents on the American side of the border when the blast happened, shaking the ground in the US Customs and Border Patrol building that he was in.

He and about a dozen other people were then told to walk across the bridge back to Canada, and were forced to leave their cars behind on the US side.

Aaron Beatty, from Cleveland, Ohio, said he crossed into Canada to see Niagara Falls on Wednesday morning.

“I said, ‘oh, I’ll just cross over to the Canadian side for a quick hour to see the other side and walk back,” Mr Beatty told the BBC.

But as he was heading back into the US, a border agent told him to go back to Canada.

“That one hour turned into almost eight hours now,” Mr Beatty said on Wednesday evening.

Other bridges connecting the US and Canada nearby – the Peace, Queenston-Lewiston and Whirlpool Rapids Bridges – were temporarily closed but had reopened to the public by Wednesday evening.