The taxi driver who tried to rescue the four police officers trapped in their burning vehicle last night said he does not know whether the doors to the vehicle were locked.

Barry McArdle described the desperate attempts to free part-time officers 39-year-old Declan Greene and 30-year-old Kenny Irvine, and full-time officers Kevin Gorman (24) and 27-year-old James Magee.

An emotional Mr McArdle, a taxi driver who came across the accident moments after it happened, said: “I would have tried the doors, but the flames were over the doors.

“I couldn’t tell you if the doors were locked. I couldn’t get near them. The only thing I could get near was the windscreen. I don’t know if anyone tried the doors.

“I hit the window, but the flames were spreading. There was only so much you could do. I saw people beating the window with metal bars or bats. I hit the window maybe three, four times with the fire extinguisher but the windows were not moving, it was having no effect and the flames were getting the better of us.”

As more details of the accident come to light, questions are beginning to mount as to how the tragic accident could have happened, as well as concerns about the suitability and safety of the vehicles. It is understood that the only way to exit the vehicle in such a situation is through the boot, and a speed limit of 45mph should be adhered to by officers.

An investigation into the horror smash is now focusing on a black box recorder that was in the 4X4, and will hold vital evidence in piecing together the final moments of the doomed journey.

Policing Board member Basil McCrea said the investigation into the accident will look at the safety of the vehicles used by officers.

It is understood the Mitsubishi Shogun spun 180 degrees before hitting the bridge, impacting mostly on the rear passenger side.