The brutal murderer of Bradford traffic policeman Ian Broadhurst will spend the rest of his life behind bars because of the gravity of the execution-style killing.

American bodybuilder David Bieber, who could have used a string of aliases during his seven years in the UK, mercilessly shot the 34 year-old dedicated officer in the head from point-blank range with a 9mm handgun as he lay on the ground helplessly wounded and pleading for his life.

Five seconds earlier Bieber had blasted PC Broadhurst in the chest, leaving him severely injured, as well as shooting at two of his colleagues, badly wounding PC Neil Roper and hitting PC James Banks' radio as they fled for their lives.

As married PC Broadhurst, of Birkenshaw, desperately begged to be spared, Bieber stood over him and fired a fifth bullet into his skull.

Yesterday a jury at Newcastle Crown Court found the doorman guilty of murdering PC Broadhurst and attempting to murder his two colleagues.

Mr Justice Moses said the aggravating feature was that Bieber did not need to shoot PC Broadhurst through the head.

He said: "You had already disabled him and he was defenceless. You could have escaped then but you chose to wait and fire a second shot at point blank range.

"It must be acknowledged he might have died as a result of your first shot but you made certain of his death.

"I regard this aggravating feature of such significance as to lead to the conclusion that the murder of PC Broadhurst was so grave that I must order that the early release provisions shall not apply and you must spend the rest of your life in prison."

The judge said the three officers were shot while doing no more and no less than performing their duties with all those qualities they had sworn to display.

"In protecting our community PC Broadhurst lost his life and his colleagues but narrowly escaped with theirs. To shoot and kill an officer in such circumstances, doing no more than serve us all, is an attack on all of us and, whilst it is a tragedy for family, friends and colleagues which we cannot share, it is a loss to us all.

"Those three officers deserve our praise and gratitude because there is no more important task imposed upon the state than to protect its citizens and that duty is fulfilled by men such as PC Broadhurst, PC Roper and PC Banks."

Bieber was given concurrent life sentences with a minimum of eight years for the attempted murders and a concurrent eight-year sentence for two firearms counts.

After the case West Yorkshire Assistant Chief Constable John Sampson described Bieber as an "enigma".

He said: "He remains a mystery. Time may help us unravel this mystery."

Detective Chief Superintendent Chris Gregg, who led the investigation, said: "The investigation into this man is still far from complete. We will be continuing our efforts to unfold and unlock the mystery. What has he been doing these last seven years?

"We ask people to look at David Bieber again. He may not have been using David Bieber or Nathan Coleman, he had any one of a number of identities."

Detective Chief Superintendent Gregg added that Bieber's actions were "beyond belief."

He said: "He is a ruthless killer who demonstrated no mercy or regard for life. Ian Broadhurst lost his life at the hands of this man and there is no doubt, in my view, that he intended to kill his two colleagues as well. They were fortunate to escape with their lives."

Det Chief Supt Gregg said the first four shots had given the officers no chance to defend themselves.

He added: "We know from the audio tape that Bieber waited some five seconds before firing the final shot at Ian's head, as he lay defenceless and injured on the ground.

"The officers, who were unarmed, could not have defended themselves against such a sudden and unexpected eruption of violence.

"Ian Broadhurst was an officer who clearly loved his job. He was not only a credit to West Yorkshire Police but also to his family and friends, who are experiencing a great loss."

He said he had nothing but admiration for the strength of character shown by PC Roper who was determined to return to the job he loved while PC Banks continued to carry out a professional job.

A statement from PC Broadhurst's family said he was a loving husband and wonderful son, brother, uncle and friend.

It went on: "He lived for his job and he paid tragically with his life for doing his duty. He left home on Boxing Day saying goodbye to his family. We never imagined it would be the last time we saw him.

"It is clear Ian and Neil posed no threat to this man. What happened in those final seconds is beyond belief and will haunt us forever.

"This year has been a long and difficult one, however we feel that at last justice has been done and we are very pleased with the verdict.

"Ian did not die in vain. He died doing the job he loved. We are immensely proud of him."

Bieber's solicitor, Christiana Hayward-Kourabas, said her client would continue to fight to clear his name.

She said: "He is confident that fresh evidence, which the judge would not give him time to submit, will establish that innocence."