The man leading the hunt for a wealthy businessman's killers has appealed to their families to turn them in.

Detective Superintendent Bill Shackleton said officers who switched their inquiries from Leeds to Bradford were closing in on the gang of professional criminals who raided the home of millionaire John Luper, leaving him dead.

The tycoon was attacked and overpowered while walking his dog close to his home in Alwoodley, Leeds.

He was dragged back to his house and his Turkish-born wife, daughter and the au pair, who were in bed, were tied up.

The terrified women managed to free themselves and raise the alarm two hours later, finding 57-year-old Mr Luper collapsed in a breakfast room.

A forensic pathologist ruled he died from asphyxiation.

The gang, thought to be four strong or more, stole cash and jewellery during the raid at about 11.30pm on February 16.

Det Supt Bill Shackleton urged the gang's families to speak out.

He said: "There will be people in Bradford and Liversedge who will have big questions in their minds about where their loved ones were on the night and morning of February 16 and 17.

"Their families will be aware that they cannot account for their whereabouts at that time. I'd urge them to come forward."

He said although a Leeds link had not been ruled out, detectives had identified a group of people in Liversedge and Bradford and believed certain individuals among them - including a woman -- could be responsible.

He said the criminals they were tracking were not just linked to robbery but also organised drug dealing.

Detectives are comparing the Luper case with other similar raids across West Yorkshire where wealthy people were targeted, tied up and had valuables stolen.

About £1,000 cash was taken during the robbery as well as Mr Luper's burgundy wallet, Mrs Luper's purse, diamond rings, earrings, a Rolex watch and two Cartier watches - one of which was distinctive because it features diamonds in the settings.

Anyone with information should contact (0113) 241 3259 or call Crimestoppers in confidence on 0800 555 111.