Gary Dobson and David Norris's conviction will 'hopefully not close the file' on murder investigation, says Mr Justice Treacy

The judge in the Stephen Lawrence trial has caused dismay among senior Metropolitan police officers by piling pressure on the force to bring the other suspects in the killing to justice.

As he sentenced Gary Dobson and David Norris to serve a minimum of 15 years and two months and 14 years and three months respectively for the "terrible and evil" murder, Mr Justice Treacy unexpectedly admonished the Metropolitan police in front of a packed courtroom.

The judge called the senior investigating officer, Detective Chief Inspector Clive Driscoll, into the witness box to tell him the Met had been rightly shamed and humbled by the Macpherson report and the failure of their investigation.

He acknowledged the Met's hard work and professionalism in recent times, but made clear that what had been achieved was "a measure of justice", and he now expected the other suspects ? Luke Knight and the brothers Neil and Jamie Acourt ? to be investigated again. There has been mention of a fourth suspect, but he has never been identified by police.

"The convictions of Gary Dobson and David Norris will not hopefully close the file on this matter," Treacy said. "On the evidence before the court, there are still three or four other killers of Stephen Lawrence at large.

"Just as advances in science have brought two people to justice, I hope the Metropolitan police will be alert to future lines of inquiry not only based on advances in science but perhaps also from information from those who have been silent so far, whoever they might be."

Publicly, the Met said nothing. But privately, senior officers, who believe the police team and the scientists involved deserved unqualified praise from the judge for their successful investigation, were angry that instead, in their view, he had sought to reopen old wounds.

It is also understood that Doreen Lawrence, Stephen's mother, was surprised by the judge's comments, having not been warned in advance. A source said she had wanted closure but the judge's comments had opened the issue up once more. She is said to believe that the chances of anyone else being brought to justice for her son's killing are almost nonexistent.

Senior Met sources told the Guardian the chance of another conviction was "very slim". One said: "We have exhausted all police lines of inquiry and scientific lines of inquiry. Frankly, if we could have found the evidence against anyone else, doesn't the judge think they would have been in the dock with the other two?"

Earlier, the judge had delivered his sentences in a crowded courtroom, watched by all 12 jurors, who on Tuesday had convicted Dobson and Norris of the murder of the 18-year-old in 1993. Cutting-edge science had unearthed new evidence in the form of a spot of Stephen's blood, fibres and a single hair that placed the defendants at the murder scene.

Extra seats were brought into court 16 for a large group of the Lawrence family.

In the public gallery, the defendants' families looked on as the judge imposed sentences under 1993 guidelines, which he admitted fell short of what could have been imposed had the murder happened more recently.

But he told the court: "There was a degree of general premeditation; it was a racist crime driven by hatred; it involved a gang of like-minded attackers; a lethal weapon was employed and known in advance to be carried; the victim was completely blameless and helpless."

After telling Dobson and Norris to remain seated, he said the murder was a "terrible, evil crime".

"A totally innocent 18-year-old youth on the threshold of a promising life was brutally cut down in the street in front of eyewitnesses by a racist, thuggish gang. You were both members of that gang. I have no doubt at all that you fully subscribed to its views and attitudes," he said.

"The crime was committed for no other reason than racial hatred ? at least one of your group was armed with a lethal knife that night. I am sure you were aware of that.

"The evidence does not prove, so that I could be sure, that either of you had a knife, but the person who used it did so with your knowledge and approval."

Dobson was later told to stand. He clasped his hands behind his back, and stared in silence at the judge as his fate was delivered. Treacy sentenced him to serve a minimum term of 15 years and two months. Norris also stood silently as the judge said he should serve a minimum of 14 years and three months. They cannot be considered for release until serving the full minimum term.

Had the men been sentenced under today's guidelines, under the 2003 Criminal Justice Act they would have received a minimum life sentence of about 25 years.

As Dobson's family left the dock his father shouted: "Shame on all of you."

After the sentencing, Doreen and Neville Lawrence briefly appeared outside court in front of a four-deep crowd of wellwishers who chanted: "Justice, Justice."

Mrs Lawrence said it had been a difficult day. "The sentences that happened may be quite low, but at the same time the judge's hands were tied. And for that, as much as he can do, I am very grateful.

"It's the beginning of starting a new life because we've been in limbo for so long. So today we're going to start moving on, and it's time to take control of my life once more." Mr Lawrence said the sentencing was "one step in a long, long journey" and called for the other killers to be brought to justice as well.

The Met will be meeting next week to decide how to proceed. The case remains open but it is likely to be wound down unless new evidence emerges shortly.

In the case of Neil Acourt and Knight, the barrier to them being brought to trial is very high. Both were acquitted in 1996 at the family's private prosecution.

The court of appeal would have to be convinced ? as in the case of Dobson ? that the Met had "new and compelling" evidence before allowing them to stand trial for the same offence again.

However, all the exhibits seized from the Acourts and Knight were subjected to the same forensic review as those of Dobson and Norris. Nothing emerged in four years to enable the Met to charge them.

Although detectives will visit Dobson and Norris in prison, neither can be compelled to see them. And under the Serious Organised Crime Act 2005, prosecutors cannot use someone as a so-called supergrass once they have been sentenced.

One possible development is a changing of allegiances leading to an ex-girlfriend or friend of one of the suspects coming forward with proof of a confession. Alternatively, either Dobson or Norris might agree to talk openly and provide evidence to pass the Crown Prosecution Service test about who wielded the knife that night in April 1993.

Jesse Jackson, the American civil rights campaigner, on a visit to the Occupy protest outside St Paul's Cathedral, said: "Justice should be swift and sure. The killing was sure but the justice is incomplete because there are still killers on the loose.

"Somebody in the community who knows where the other three are has some moral obligation to let their conscience be their guidance and speak out."