Downing Street aide Ruth Turner has been interviewed again as part of the police cash-for-honours investigation.
Ruth Turner has denied any wrongdoing
Ms Turner was arrested in January and questioned in connection with the honours probe and on suspicion of perverting the course of justice.
She answered her bail on Tuesday and was interviewed for a second time.
Scotland Yard is investigating whether peerages were awarded by political parties in return for money. All involved deny any wrongdoing.
Ms Turner, one of the prime minister's closest aides, is director of government relations and a vital link between Downing Street and the Labour Party.
She was first questioned by police in September, then in January arrested, interviewed and released on bail.
'No running commentary'
While answering bail on Tuesday, Ms Turner was questioned again by police for about two hours.
Asked about the development, Downing Street said they would not give "a running commentary on a police inquiry".
But the prime minister has previously given Ms Turner his full backing, describing her as "a person of the highest integrity for whom I have great regard".
Scotland Yard refused to confirm the name of the woman interviewed on Tuesday, but identified her as a woman previously arrested at her home on 19 January, the day Ms Turner was arrested at her home.
Since Ms Turner's arrest in January, Labour's chief fundraiser Lord Levy has also been arrested and interviewed on suspicion of conspiracy to pervert the course of justice. He was later released on bail.
Neither has been charged with any offence.
The arrests appear to show the original police inquiry has widened, to investigate whether there has been an attempted "cover up".
The original police inquiry began after it emerged that large secret loans were made to the Labour Party before the 2005 general election, and that some lenders were later nominated for peerages. But it has since widened to cover the other main parties.
Scotland Yard had been expected to hand over a file on the case in January to the Crown Prosecution Service, to decide whether any charges should be brought.
But a police spokesman said in January that the investigation - during which more than 90 people have been questioned, including Tony Blair twice - needed more time.