Libya's interim leaders know the whereabouts of a suspect in the murder of PC Yvonne Fletcher, reports say.
The Daily Telegraph says Matouk Mohamed Matouk is the last named suspect alive connected with the killing outside the Libyan embassy in London.
The paper quotes Ali Tarhouni, a senior National Transitional Council (NTC) figure, saying "we know where he is".
PM David Cameron said he was sure the NTC would co-operate fully with Scotland Yard's ongoing investigation.
Mr Tarhouni denied reports that Mr Matouk had been placed under house arrest due to his long service with the Gaddafi regime, the Telegraph said.
It described Mr Tarhouni as the de facto deputy prime minister of the new government and said Mr Matouk had recently tried to defect to the NTC.
The paper said the disclosure that rebels knew where Mr Matouk was, would pile pressure on Libya's new government to co-operate with Scotland Yard.
A Metropolitan Police spokesman said it remained committed to identifying those involved and detectives were still in regular contact with PC Fletcher's family.
The British government has said it intends to send police officers to Libya to continue their investigation into the killing.
Although there is no agreement that allows Libyans to be extradited to Britain, Conservative MP Daniel Kawczynski, chairman of the all-party Libya group, has called for Mr Matouk to be returned to Britain to face justice.
Mr Cameron said: "Obviously I want justice for Yvonne Fletcher's family. There's an ongoing police investigation and I'm sure the new authorities in Libya will co-operate fully with that investigation."
He said her murder was a reminder of the "horrors" of the Gaddafi regime which Britain had played a proud part in bringing to an end.
Mr Matouk has previously presided over Libya's nuclear programme, liaising with the International Atomic Energy Agency, the Telegraph says.
He is not the only suspect in the killing of PC Fletcher.
Abdulqadir al-Baghdadi is one of three other former diplomatic staff alleged to have been involved, according to a witness statement given to UK prosecutors.
Libyan opposition officials revealed on Tuesday that the "co-conspirator" had been found dead, and Mr Hague said the UK wanted to know "a great deal more" about the circumstances.
Documents handed to the Crown Prosecution Service also alleged that Abdulmagid Salah Ameri, a more junior diplomat at the time, had actually fired the gun.
He is believed to have died some time ago.
PC Fletcher was shot dead when a gunman fired from the embassy at a protest outside.
About 30 Libyans were allowed to leave the embassy under diplomatic conventions after an 11-day stand-off following the shooting.