The family of a woman killed in a motorway crash which left investigators "scratching their heads" say they still have many unanswered questions following an inquest into her death.

Catherine Gardiner was driving a Ford Transit minibus along the M4 at junction 14 near Newbury before it decelerated rapidly and a lorry crashed into it from behind.

She died alongside two passengers, Jay Aleixo and Lorraine MacLellan.

READ MORE: Survivor describes moment lorry crashed into Thatcham school minibus - killing three staff members

At an inquest at Reading Town Hall on Thursday, coroner Heidi Connor recorded a conclusion of road traffic accident but said the reasons why their vehicle stopped remained unclear.

In a statement following the inquest, Miss Gardiner's son Ben Stubbs said they had hoped the hearing would "fill in some of the blanks".

"Although we welcome the coroner's findings, we still feel there are many questions that have been left unanswered," he said.

"It has been almost a year to the day since mum died and one of the hardest things for us has been not knowing exactly what happened," he added.

Ms Connor said the "missing piece of the jigsaw" was why the vehicle went from 50mph to 0 within seconds.

Police and car analysts said they could not find a fault with the engine, and were unable to find any evidence of human error in the crash on October 11, 2018.

Ms Connor said "based on the balance of probabilities" it was more likely an issue with the vehicle.

She had earlier said: "We are collectively scratching our heads.

"We cannot explain from evidence that I've heard which would explain (why) the vehicle reacted as it did, starts juddering, and comes to a halt."

Lorry driver Graham Scivier said he had "seconds to react" after the minibus suddenly appeared in front of him.

The vehicle contained staff members and students from a specialist autism school, Prior's Court in Thatcham. Three students from the school were also injured.

Mr Aleixo, 44, from Thatcham, and Ms MacLellan, 60, from Farnborough, died at the scene, while Miss Gardiner, of Reading, died at John Radcliffe Hospital the next day.

Ms Connor added that she would be writing to Ford to carry out a further examination of the vehicle for more answers, after they examined the engine.

"When taking into account the driver's injuries, the lack of road markings, and witness evidence, a problem with the vehicle remains more likely than driver input," she said.

Adriaan Moolman from Ford, who analysed the vehicle after the crash, told the inquest he had "absolutely no evidence" there was anything wrong with the engine.

Similarly, Ben Matthews, forensic vehicle examiner for Thames Valley Police, said analysing the crash had been a "struggle".

He said: "If I'm brutally honest, we've explored all the options. I can't bring anything new to the table."

The coroner also acknowledged that the brake lights of the minibus had turned on - but only at the last second before the collision.

Patrick Maguire, a specialist serious injury lawyer at Slater and Gordon who is representing the family, said after the inquest: "Catherine was an experienced driver who always took great care on the roads and the sudden and unexplained nature of her death is something her family believe needs further investigation to help them understand and come to terms with what happened."