Midland Metro was at the centre of four investigations last night following a collision between two trams which saw at least 13 people taken to hospital with injuries.
About 40 people were led from the trams, which collided near Benson Street station in the Jewellery Quarter area of Birmingham.
It is thought that one of the trams went into the rear of the other as they were heading towards the city centre. It is not known whether one of the trams was stationary.
The passengers had minor injuries although witnesses described them as looking "shocked and scared" as they were led up the track following the accident, which happened just before midday.
Midland Metro, which is managed by Travel West Midlands through a separate division, Travel Midland Metro, is now being investigated by the Office of Rail Regulation and the British Transport Police, who both have powers to prosecute.
The Rail Accident Investigation Branch (RAIB), which is part of the Department for Transport, is also looking into the circumstances surrounding the collision. Midland Metro are also carrying out their own inquiry.
The two trams remained on the tracks yesterday until late afternoon as transport police officers examined the scene of the accident.
The bumper of one of the trams – named John Stanley Webb – was twisted, and the front of its carriage mangled and windows cracked.
The rear bumper of the other tram – named Jeff Astle – was also mangled. The back window of the vehicle was smashed.
Adam Wilson, a 35-year-old window fitter who was working at a nearby house in Devonshire Street, said he heard a "very loud bang" before he rushed to the track side.
He said: "The passengers were left in the tram for at least half an hour until quite a few paramedics turned up with police officers.
"When they were taken off the tram and brought up the track they looked shocked and scared."
Travel West Midlands divisional corporate affairs director Phil Bateman said services were only running between Wolverhampton and Wednesbury after the collision, although the Birmingham service resumed last night at 7.30pm.
Mr Bateman said he would not speculate on what happened, adding: "We are carrying out our own internal engineering investigation but we are quite confident that that system remains safe."
British Transport Police chief inspector Allan Gregory said it was too early to see if any prosecutions would take place.
He added: "It is a safe system to my knowledge. We have interviewed the passengers because we need to know what has happened – whether it was a technical issue or a human issue."
A spokesman from the Office of Rail Regulation, which took over the responsibilities of the Health and Safety Executive on the railway in April, said the independent body did have an investigator at the site of the collision.
"We will see if there has been any breaches of rules or regulations and that is why we have someone on site at the moment," he added.
A spokesman for the Department for Transport said RAIB would not seek to apportion blame for the accident.
He said: "Our reports say what the issues are behind collisions, what causes them, and if there were any long term factors in place."