British Transport Police have issued an appeal for five "mystery" railway workers seen on the track hours before the Potters Bar train crash.

Seven people were killed and 76 injured in the crash on 10 May, which has been blamed on faulty points.

Five people wearing orange rail worker jackets were captured on CCTV and seen by other workers at 0825 BST - more than four hours before the crash.

And no-one from Railtrack or Jarvis, the contractor in charge of maintenance at Potters Bar, knows who they are.

We need to establish what they were doing there and find out what they can tell us Chief Supt Crowther

Chief Superintendent Paul Crowther, of the British Transport Police (BTP), said investigations had so far failed to trace the men.

He said: "It's an important part of the inquiry to talk to everyone who was at the area or who has done work in the area.

"We are continuing to make inquiries with Railtrack and Jarvis but so far, we have been unable to trace these five men.

"There must be people within the industry who know something about this and we would very much like to hear from them."

'Sabotage'

He added: "It's quite possible that they were doing something completely unrelated but we need to establish what they were doing there and find out what they can tell us."

A police spokesman said it was strange these people had not come forward before.

The BTP spent several weeks trying to trace the men through Railtrack and Jarvis, but without success.

The sighting of the five men was revealed in a Railtrack memo seen by the BBC.

Investigations have focused on the points

BBC transport correspondent Tom Symonds said it was a "significant development".

He added: "It could possibly be rail workers up to no good and deliberately sabotaging the line.

"Or it could be another gang working on a different section of track and just happened to be in the area."

A Jarvis statement said: "We are providing the BTP with all staff transit movements in respect of the period when these individuals were seen.

Public inquiry

"Our records account for all of our staff and we have found no record of any Jarvis personnel on or near the site at the time in question.

"No maintenance or other work was scheduled in this area for the time in question."

A preliminary investigation into the tragedy revealed that loose nuts on a set of points just before the station was the likely cause of the derailment.

The last carriage of the 1245 West Anglia Great Northern King's Cross to King's Lynn service came off the tracks as it passed over them.

The crash shook traveller confidence

The Health and Safety Executive said the points broke because of the loose nuts, and are conducting forensic tests to establish the cause.

Jarvis claims the points were sabotaged but the BTP said that was still unlikely.

And some relatives of victims have said they believe the accident was caused by poor maintenance.

Former Transport Secretary Stephen Byers said he would await the Health and Safety Executive's full report before deciding whether to seek a public inquiry.

A team of 50 officers is working on Operation Declare and has questioned 2,100 people, taking 350 statements.