The Metropolitan Police chief who led the cash-for-honours inquiry said he received "less than full co-operation" from some people involved in the probe.
Mr Yates: Some treated allegations as political rather criminal problem
Assistant Commissioner John Yates also told MPs "political pressure", but not improper pressure, had been put on him.
He denied that the 16-month inquiry which ended in no charges being brought had been a "wild goose chase".
During a two-hour grilling he said such allegations were difficult to prove as they were "bargains made in secret".
The Commons public administration committee is looking at the "lessons learned" from the investigation, such as whether the law needs changing.
The police studied claims that peerages had been offered in return for loans or donations.
The MPs who questioned Mr Yates were Carmen Dowd, head of the special crime division of the Crown Prosecution Service (CPS), and David Perry QC, adviser to the CPS.
Mr Yates said the £1m probe had taken a long time because the subject was "necessarily quite complex and difficult".
"There were, however, instances when we received less than full cooperation," he said.
"I don't say that now in a sense that it was deliberate in its intent. But I think there was a sense they thought that we would ask questions get some answers and simply go away. That is not how police investigations work.
"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."
Mr Yates said he had only discovered in January this year how the relevant honours list had been put together.
He said the Cabinet Office had co-operated in full throughout the investigation, but asked who had not co-operated, he replied: "I think it would be quite obvious to all people who that was."
Asked if Downing Street had co-operated, he said he had learned that "Downing Street" has a number of meanings.
Tory MP Charles Walker pressed the police chief to say why it was necessary to beat down the door of former Number 10 aide Ruth Turner in a "6am raid".
Mr Yates said, in general, people under investigation for conspiracy to pervert the course of justice could not be put "on notice" by the police in advance.
"People under suspicion for this sort of case will try to hide evidence," he said.
Mr Walker asked if anyone on his team had leaked to the press details of the people involved in the case or when raids on their homes would occur.
"It seemed the press knew where and when to turn up," he said.
Mr Yates said this was "absolute nonsense", asking: "Have you got proof of that?"
He said the police in this case did not "beat down the door" and he had looked at the "least intrusive way" of dealing with some "extremely high profile" people.
He denied he had a "cavalier" attitude towards suspects and witnesses in the investigation.
"Throughout this inquiry I worked, from day one, very closely with the Crown Prosecution Service and very closely with counsel. We sought their advice throughout," he said.
"We challenged ourselves and I challenged them - do we continue? Is it right? Is it proper? Is it proportionate? So it's not just me going off on a wild goose chase thinking this is great fun because it wasn't great fun at all, it was bloody difficult."
Mr Yates was asked whether a prosecution could ever be successful under present legislation - the 1925 Honours Act - and whether the standard of proof required was too high.
"These type of cases are very, very difficult to prove because they are bargains made in secret," he said.
"Both parties have an absolutely vested interest in those secrets not coming out."
Asked if he believed there was a "trade in peerages", Mr Yates replied: "I think I've done my job. I followed the evidence. I provided that evidence to the CPS and they made their decision. I don't think I should comment further than that."
The MPs began a general inquiry into "propriety issues" relating to the honours system in March last year.
But they had to suspend it just a week later when the separate police investigation was launched.
'Thorough and exhaustive'
The police investigation, during which more than 130 people were interviewed and four people were arrested, focused on allegations that peerages had been offered in return for loans to Labour and the Conservatives ahead of the 2005 general election.
Figures questioned by officers included the then Prime Minister Tony Blair and former Tory leader Michael Howard.
All involved in the investigation denied any wrongdoing and the CPS said in July that there was "insufficient evidence to provide a realistic prospect of conviction against any individual for any offence".
The MPs decided to resume their inquiry after the CPS announcement that there would not be any charges.
Following the questioning of Mr Yates by the committee, Foreign Secretary David Miliband told the BBC: "All I know is that the independent prosecuting authorities decided there should be no charges and as far as I'm concerned that's the end of the matter and we can go on with the serious business of government."