Officers arrested the man, believed to be in his 20s, at an address in the Pitsmoor area of the city, close to where 53-year-old Mr Khan was killed in a hail of gunfire while driving his cab in the early hours of last Wednesday morning.

Detective Supertintendent Martyn Bates, who is leading the inquiry, said between 40 and 60 officers had been working on the case and added that the focus of his investigation was on the neighbourhood where the killing took place.

"We arrested the fourth man during the night between Monday evening and Tuesday morning and he was questioned and released on police bail," said Det Supt Bates yesterday.

"At the moment all I am prepared to say is that the focus of the investigation is very local. A lot of our house-to-house inquiries are concentrating on people who live in the area, although that is not say we may not progress outside Sheffield at some stage.

"We have been looking at a number of individuals and previous incidents that may have led to Mr Khan's death.

"I am aware of the suggestion of a feud and we are pursuing one key motive which has been suggested to us by a number of different sources. But I still maintain that Mr Khan was a hard-working family man."

Mr Khan was a father-of-five and last weekend his children released a statement appealing to the public to help the police in the search for their father's killer.

Last night, a week on from the incident, officers from South Yorkshire police mounted an operation in Scott Road, where the shooting took place.

They stopped motorists in the area between 11pm and 2am in a bid to track down potential witnesses.

Det Supt Bates added: "Hundreds of people use Barnsley Road in the area of Scott Road and may not even realise that they were in the area at the time of the murder.

"We are looking for anyone who saw somebody running in the area or driving a car fast or in an erratic manner. Any information could be vital to the investigation."

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