Five separate investigation were underway yesterday into the Luas/bus crash in Dublin which has left a teenage girl fighting for her life.

The mystery of the smash deepened yesterday as film footage appeared to show the double decker had a green light.

The 18-year-old girl is in a critical condition in Beaumont Hospital after undergoing emergency surgery for head injuries sustained in the collision.

Preliminary investigations into the crash are understood to show the No 16 bus had a green light as it travelled down O'Connell Street, where the accident happened on Tuesday afternoon. CCTV footage from a No 46 bus travelling behind the No 16 show the vehicle had a green light just before it was struck, it is believed.

However, the possibility that the incident was as a result of a light system error, and not as a result of driver error, is also being examined.

A spokeswoman for Veolia, which operates the Luas, said the company's investigation is underway and she would not be making any comment.

The critically injured young woman, believed to be from the Beaumont area of the city, had first been rushed to the city's Mater Hospital after being one of at least 24 passengers taken from the scene.

But after assessment she was transferred to Beaumont Hospital which specialises in head and brain injuries. The hospital this morning confirmed the girl was in a critical condition.

It is unknown if the girl was a passenger on the Luas Red Line tram, which was travelling towards Connolly Station from Abbey Street, or the Number 16 bus that was travelling northbound from O'Connell Bridge to Parnell Square.

Sources have also indicated it is possible the young woman was a pedestrian who was unfortunately caught up in the accident.

She is one of four people who remain in hospital following the incident.

Two of those — the driver of the Luas tram and a bus passenger — are being treated in St James's Hospital and the fourth is being treated in the Mater.

The bus passenger in St James' is believed to be a woman who sustained two broken legs. The hospital would not comment on the patients.

Over 20 people were admitted to hospital following the crash, which happened just before 3pm yesterday.

The majority of the injured were discharged a short time later, including the driver of the No 16 bus.

A Mater spokesperson said: “We have one woman who remained here overnight who is in a serious but stable condition.” The extent of her injuries are not known.

Following the accident, witnesses said some terrified passengers were trapped in the Luas for a short time as the door jammed.

One witness reported: “The doors wouldn't open when it crashed, so everybody was panicking to get the doors open — to break a window or anything.”

The driver of the tram had to be cut out of his cabin after the front of the Luas was crushed.

“They had to cut him out but he was conscious andwas speaking to the emergency services,” according to one eyewitness.

He was rushed to hospital, while two bus passengers were wedged for more than an hour on the first floor of the bus near the stairwell.

The CCTV footage shows the two buses, the No 16 and the No 46B, travelling almost side by side down O’Connell Street. A woman cyclist was travelling in front of the bus and a white van was travellingin front of the cyclist.

The van indicated to turn left down Middle Abbey Street and, as a result, the cyclist slowed down, causing the No 46B to come to a halt as well. At this point, the No 16, which was not impeded in the outside lane, passed through the junction on a green light, the footage from the 46B shows.