Marc Malpas and Aaron Matthews died in the crash

They were named locally as driver Natalie Malpas, 20, her brother Marc, 16, and their friends, brothers Aaron and Gavin Matthews, aged 16 and 19.

Their Renault Megane careered into a garden wall at 2200 BST on Wednesday.

Dyfed-Powys Police said they were looking at "all possible causes" for the crash on a straight section of the Pontamman Road.

The group had been playing pool at a local pub before the accident.

Marc's 15-year-old girlfriend Rachel Meed said she had seen him minutes before the crash.

After laying flowers at the scene, she said: "The last time I saw him he just said 'I love you and I'll see you later on'."

Rachel's mother Jeanette said: "He was a lovely boy - very thoughtful. He spent so much time with us over the last couple of months, I just expect to see him walking past our window."

She said that Marc's sister Natalie, who police confirmed was driving the car, has a young son.

The family are originally from Kent, while the Matthews brothers were originally from the Bridgend area.

Debris was scattered in neighbouring gardens. David Harries' bungalow took the full force of the crash.

I feel sorry for them and their parents - it's tragic David Harries

"I was just sitting down watching the news. The football had just finished and the front room blinds were open. The next thing I knew, there was an enormous bang and all I could see outside was dust everywhere," he said.

"My first reaction was one of utter disbelief. This morning it's the same - I'm just shocked. We've been campaigning locally to get something done about this road for months and then something like this happens, confirming all our worst fears."

Mr Harries, 58, said he had only just finished compiling a petition about speeding on the road.

As spokesman for a local campaign, he typed a letter on Wednesday calling for urgent action, which he intended to send on Thursday to the county council traffic department and local police.

Police said all four people were pronounced dead at the scene

He said there had been two accidents on the stretch of road in the last month and residents had written to police before.

"This should not happen on a straight road. Cars and motorbikes are speeding - it's like the Isle of Man," he said.

Insp Tom Edwards said that it was at the very early stages of investigation but they were "looking at all possible causes".

He said speeding as a factor could not be ruled in or out. He said there had been four accidents that resulted in injuries on that stretch of road in the last five years.

Police said families had been informed but they have not officially named the victims.

They are anxious to trace the movement of all youngsters leading up to the accident.

Witnesses appeal

Carmarthenshire Councillor Lyn Llewellyn, who lives nearby, said the road was not known to be dangerous although there had been complaints over speeding.

He said: "It's difficult to imagine how this could have happened. The road is certainly straight.

"There have been some incidents on this piece of roadway but I don't think there have been any fatalities."

He said people in the area had complained about speeding along the stretch of road as motorists drove out of Ammanford towards Glanaman.