Two of Tony Blair's closest aides have been re-bailed by police as part of the cash-for-honours inquiry.
Lord Levy is the Labour Party's chief fundraiser
No 10 fundraiser Lord Levy and Downing Street aide Ruth Turner returned to police on Tuesday. All involved with the inquiry deny any wrongdoing.
Both had been arrested and quizzed in the probe which was initially into the sale of honours but later broadened to investigate any cover-up attempt.
Prosecutors are still considering the police file from the year-long probe.
On Monday it emerged the Crown Prosecution Service had asked police, who interviewed 136 people during the course of the investigation, to carry out further inquiries before deciding whether any charges should be brought.
The investigation began a year ago, after it emerged that secret loans had been made to Labour before the 2005 general election, and that some lenders had subsequently been nominated for peerages.
The inquiry was widened to cover the other main parties. Among those questioned as potential witnesses were Tony Blair and former Conservative leader Michael Howard.
Scotland Yard confirmed on Tuesday that the investigation had cost more than £750,000 so far.
Lord Levy was first arrested in July 2006, in connection with alleged offences under the Honours (Prevention of Abuses) Act 1925.
He was re-arrested in January and questioned on suspicion of conspiracy to pervert the course of justice.
Ms Turner was first questioned by police in September 2006, then was arrested in January and questioned about honours allegations and on suspicion of perverting the course of justice.
Two other people have been arrested in connection with the inquiry. Labour donor Sir Christopher Evans was re-bailed on Monday, while head teacher Des Smith has been told he will not be charged.