IT was a crime of depravity that shocked an entire city.

Now the starkest question being asked by the community left reeling in the wake of the rape of a teenage girl on a bus in Glasgow is: "How could this happen here?"

The 14-year-old was travelling with a friend on a number 57 bus on the south side of the city when she was raped by two men before other passengers could come to her rescue. The assault is believed to have taken place while the bus was in the Pollok area of the city.

Local residents expressed their shock at the attack.

"It is a terrible story," said Ann Sampson, who lives near where the attack took place. "But it's hard to understand how this could happen on a bus in the evening when there were other passengers on board."

Last Friday night, two 14-year-old girls boarded the bus as it left the Silverburn shopping centre at around 10.30pm.

One girl remained downstairs while the other went upstairs where she was approached by two men, thought to be aged around 18 and 20. Her friend went to the upper deck to see her and, when she saw what was happening, raised the alarm with passengers downstairs.

A woman and two men came to her aid and they left the bus two stops away from the shopping centre.

The incident is believed to have lasted about 10 minutes.

The passengers waited with the girls until another bus came and put them on it to travel home, thought to be in the Darnley area of the city, where police were contacted.

Although some First Glasgow buses have CCTV, the operator said that no footage is available from the bus on which the attack took place.

Raising many of the concerns shared by locals, Pollok resident Sampson asked: "Why did no-one call the police? Why were the girls left to get another bus home?"

John Wilson, a 53-year-old from Darnley, said: "It's the fact it happened so fast. For that all to go on within two stops, it beggars belief. I'd want to think my daughter and her pals were safe on public transport. I suppose this kind of thing is very rare but I'll think twice about letting her go on the bus now."

The police officer in charge of the case, DC Jackie Carroll, said the girl had been left "traumatised", and investigators hope the passengers who came to her aid will come forward to help with enquiries.

The attackers remained on the bus for a further two hours after the attack, before being ejected near Queen's Park at Pollokshaws Road at around 12.30am. They are said to have become increasingly loud and were making lewd and crude comments to other passengers.

First Glasgow insists the driver knew nothing of the attack.

A company spokesman said: "In the case in question our driver was not aware of the assault. Unfortunately, the incident took place on the top deck of the bus and therefore out of our driver's eyeline, and no-one on board approached our driver to report a disturbance. The driver ejected the two males in question later in the journey – this was due to separate unruly behaviour.

"We continue to assist the police with their enquiries and will do everything we can to help them catch the perpetrators. In the meantime, our thoughts are very much with the victim of the assault."

The spokesman said all drivers are trained to deal with incidents onboard and, with 114 million passengers travelling in the city each year, serious incidents are very rare.

He added: "Bus travel is an extremely safe form of transport."

Sandy Brinley, of Rape Crisis Scotland, said the charity would not comment on individual cases.