The police investigation to find the killer or killers of Emma Caldwell is believed to have focused on Glasgow's Turkish community.

The four men who were arrested yesterday in connection with the 27-year-old prostitute's death more than two years ago are understood to be Turkish or Kurdish.

A kebab meat processing premises in Duke Street in the city's east end was raided and sealed off.

A Turkish community centre in Bridge Street near the city centre was also raided and closed while officers searched the building.

In March 2006 the three-storey community centre, which is particularly popular with private taxi drivers who often park outside it at night, was sealed off and searched by police hunting Miss Caldwell's killer.

It is believed a car was then removed for forensic examination and several people questioned, with police sources having previously informed The Herald that a Turkish immigrant was "of interest" to police investigations.

Around 50 Skoda Octavia cabs, similar to the one a woman believed to be Miss Caldwell was caught on CCTV getting into, were impounded after police issued warrants following an appeal in 2005.

Recent reports have also speculated that Turkish or Kurdish men were suspects for the murder, with some claiming more than a dozen men with links to the Turkish separatist PKK have been questioned by detectives.

Last year it was also claimed that a Turkish immigrant who disappeared a week after Miss Caldwell vanished from the streets of Glasgow was being hunted by detectives.

The suspect, understood to have been granted asylum in Britain in 2003, was last seen in Scotland days after Miss Caldwell went missing.

Information passed to the police by a benefits office worker helped confirm the suspect's movements before he is believed to have fled the city.

He is alleged to have links to Glasgow's vice trade, although it is not known if the man was ever brought in by police on the case.

Ziggy Zahir, 23, who owns the shop next door to the Duke Street meat processing plant, said: "I arrived here about 7.30 in the morning and there was loads of police and the place was taped off.

"I asked what was going on and they told me they couldn't say but told me I could open as normal.

"The premises was used as a Turkish doner kebab meat processing factory and it's been there for years. About a month ago it closed down and I heard they had moved to another premises in Glasgow."

Others operating businesses close to the community centre claim it has been visited by police on numerous occasions in the past two years.

A murder hunt was launched in May 2005 after a dog walker discovered Miss Caldwell's body in woods near Roberton, Lanarkshire.

She became addicted to heroin after the death of her sister, and turned to prostitution to feed her habit.

She was last seen at around 11pm on April 4 near a women's hostel in the Govanhill area of Glasgow.

Police made repeated appeals to the public to find her killer, at one point projecting a 60ft image of her on to a tower block in Gorbals district.

An appeal was screened on the BBC's Crimewatch programme, and a £10,000 reward was offered by Crimestoppers.

In July a friend of Miss Caldwell's said she remembered visiting a property in Carluke, South Lanarkshire, in the months before her friend's murder. The town is not far from the spot where Miss Caldwell's body was found.

The friend has given several statements to the police and is understood to have looked at more than 250 photographs of men who potentially took her and Miss Caldwell to Carluke.

Earlier this year prostitute Susan Shivers was killed just yards from where Miss Caldwell was last seen alive. She had been stabbed and left for dead.

Since 1991 seven other prostitutes - Diane McInally, Karen McGregor, Leona McGovern, Marjorie Roberts, Jackie Gallagher, Tracey Wylde and Margo Lafferty - have been found murdered.