Canadian diplomats in Afghanistan were ordered in 2007 to hold back information in their reports to Ottawa about the handling of the prisoners, say defence and foreign affairs sources.
The instruction - issued soon after allegations of torture by Afghan authorities began appearing in public - was aimed at defusing the explosive human-rights controversy, said sources who spoke on the condition of anonymity.
There was a fear that graphic reports, even in censored form, could be uncovered by opposition parties and the media through access-to-information laws, leading to revelations that would further erode already-tenuous public support.
The controversy was seen as "detracting from the narrative" the Harper government was trying to weave around the mission, said one official.
"It was meant to put on happy face," he added.
The instruction was passed over the telephone by senior officials in the Privy Council Office and reinforced in follow-up conferences between Ottawa and Kabul, as well Ottawa and Kandahar, sources said.
Throughout 2006 and in early 2007, diplomat Richard Colvin wrote several blistering reports that were widely circulated in both the foreign affairs and defence departments that warned of torture in Afghan jails.
Mr. Colvin will be the centre of attention Wednesday as the House of Commons special committee on Afghanistan holds its second public hearing into the country's handling of prisoners in the Afghan war.
He is expected to testify about when soldiers and diplomats became aware of the abuse, which even though perpetrated by Afghans, still implicates Canada in possible violations of international law in turning over prisoners to the Afghans.
Senior members of the government, from Prime Minister Stephen Harper on down, say they never saw Mr. Colvin's warnings in 2006, and Defence Minister Peter MacKay promised last month to investigate how far up the chain those reports went. It's unclear what, if anything, he has uncovered.
Liberal defence critic Ujjal Dosanjh said the Conservative government was not interested in discovering the truth in 2007 and still isn't.
"This shows that government from its core and highest positions was engaged in deliberately not wanting to hear, see or speak about torture allegations," Mr. Dosanjh said late Tuesday.
"Now a deliberate cover up continues."
The Commons investigation was called after a separate, but more limited probe, by the independent Military Police Complaints Commission was sidelined by legal wrangling. The commission's head, Peter Tinsley, will also be testifying before MPs on Wednesday.
Mr. Colvin was subpoenaed to appear before the watchdog commission's inquiry, but federal lawyers tried to get him stricken from a witness list and imposed a national security gag on much of his testimony.
He won't face that restriction in Parliament, which grants immunity to those who testify before committees.
The police commission launched its investigation after two human-rights groups complained that Canadian troops should not be handing over prisoners to a system that notoriously practises torture.
The agency only decided to hold public hearings in 2008 after the Foreign Affairs Department refused to hand over documents related to the controversy. The Canadian military had already co-operated by handing over thousands of records.
A series of requests by The Canadian Press under access to information laws sent to the Privy Council Office, Foreign Affairs and Corrections Canada have been returned with either "no records" found, a handful of heavily censored documents - or were ignored in violation of federal law.
In particular, the correctional service said in 2007 it couldn't release records because it was consulting another department involved in the Afghan file. The documents still have not been released, even after a complaint to the country's information commissioner.