Two staff from Alton Towers are under investigation over the horrific Smiler ride crash that seriously injured four people, including two young women who each lost a leg.

The Mail on Sunday has learned that human error and a failure to follow basic safety procedures led to the devastating accident.

An engineer overrode an automatic safety lock, allowing the ride operator to send the carriage full of people careering into an empty vehicle that had ‘stalled’ further down the track.

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Terror: How the tragedy unfolded, with an engineer overriding the system and the operator pushing the restart button

Our revelations – particularly the fact that staff are able to perform manual overrides at all – will raise disturbing questions about the safety features on the Smiler and other rides.

This newspaper understands that despite the blunder, both employees involved are still working at the park, though in different roles, while a criminal investigation by the Health and Safety Executive (HSE) is under way.

Student Leah Washington, 18, had her left leg amputated after the collision on June 2. Boyfriend Joe Pugh, 18, had both knees smashed. Dancer Vicky Balch, 20, had her right leg amputated below the knee three weeks afterwards.

Sources at the Staffordshire theme park told this newspaper how problems earlier in the day on the Smiler had resulted in an engineer being called.

These accounts tally with our exclusive interviews with Leah and Joe, who both recalled boarding a carriage when they reached the head of the queue, then being asked to disembark so the vehicle could be sent round without passengers.

Crucially, that empty carriage never completed a circuit, possibly because of high winds, and failed to make it up the ‘bat wing’ section of the ride, rolling back down the slope and coming to a rest.

Trapped: Joe and Leah were trapped in the carriage for hours following the crash in June

Inspirational: The couple are bravely facing the future together despite the life-changing injuries

Brave: Leah Washington, 18 (centre) from Barnsley posed for this picture after having her leg amputated

The most fundamental safeguard on the German-built ride is that two carriages should never be able to travel on the same section of track because sensors trigger braking devices to prevent it.

When the 16 passengers, including Leah and Joe, were loaded into a carriage for the second time, it was halted at the top of the first upward section as the automatic brakes kicked in.

The computerised safety system was working perfectly as the sensors were being told there was another carriage on the same section of track, even if operating staff were inexplicably unaware of it.

The teenagers recalled being held at the top of the section for about ten minutes, but did not know why. Joe said: ‘It was really windy and we were freezing up there.

‘I said to Leah, “Something’s not right here – this shouldn’t be happening.” ’

Meanwhile, an engineer engaged a manual override, allowing the carriage with passengers to continue once the ride operator had restarted the system.

One Mail on Sunday source within Alton Towers said: ‘The engineer should never have manually overridden the safety lock with people on the track.’