The Scotland Yard detective who led the "cash for honours" inquiry said that he had been treated as a "political problem" by some of the people under investigation.
Assistant Commissioner John Yates, whose inquiry looked into claims that peerages were awarded in return for political donations when Tony Blair was British Prime Minister, said that they had not always received full co-operation.
Giving evidence to the resumed parliamentary inquiry into the affair by the Commons Public Administration Committee, he said: "They thought we would ask questions, get answers and simply go away. That is not how police work."
Assistant Commissioner John Yates
He said that a central challenge of the case was establishing how the 2005 House of Lords appointments list was drawn up, but it was not until January of this year that they found out "how that list was put together".
Mr Yates said: "I don't think people deliberately misled us, but I do think in retrospect and with hindsight we were treated as a political problem, not a criminal problem."
He said that the police received full co-operation from "certain aspects," adding that the cabinet office co-operated in full throughout the investigation.
Asked who did not co-operate with the investigation, Mr Yates said: "I think it would be quite obvious to all people who that was."
Asked whether Downing Street co-operated, he said he had learned that "Downing Street" has a number of meanings.
He added that although he felt "uncomfortable" on several occasions, he had never come under "improper pressure".
The committee announced in July that it was resuming its inquiry — which was supended pending the police investigation — after the Crown Prosecution Service announced there was insufficient evidence to bring charges.
In an opening statement to the committee, Mr Yates said it would be "wholly inappropriate" to reveal personal accusations made against individuals who had no right of reply.
He said that representatives of some of the witnesses and suspects involved in the inquiry have contacted him to express concern that any information they provided must remain confidential.
Mr Yates said that he recognised the "enormous public interest" generated by the controversial 19-month inquiry.
"While I cannot divulge the details of the evidence against the individuals, I can, I believe, talk about the case in a generic sense," he said.
"In a way that will not breach the duty of confidentiality of those who were dealt with in this case."
PA