Canada is still unable to account for at least 50 prisoners it captured and handed over to Afghan authorities, keeping alive concerns that some detainees could have been subject to torture.

Canadian sources tell The Globe and Mail that the prisoners may be unaccounted for because of shoddy record-keeping by Afghan officials -- indicating the detainees have likely returned home safely.

Another explanation is that many prisoners often buy their freedom from corrupt jails, meaning no record would have been made.

The Canadian sources said they have not received any indication that the missing detainees were tortured or killed inside Afghan jails.

However, officials familiar with Afghanistan's prisons say foul play must not be dismissed.

"There are lots of possible explanations for how people get lost in the detention system," a Western official told the newspaper. "Some are benign, others much less so."

Following reports of torture earlier this year, Ottawa asked for a full accounting of approximately 200 detainees handed over to Afghan authorities by Canadian forces before May 3, 2007.

Months later, a quarter of those 200 detainees remain missing -- having not been listed as either released or still in custody.

Under a re-written prisoner-transfer agreement reached with Afghanistan on May 3, Canada has since been granted greater access to captured insurgents.

Among provisions of the new deal is a guarantee that captured fighters can be interviewed in private without the intimidating presence of their Afghan jailers.

Ottawa negotiated the deal despite insisting that the allegations of torture were false.

Chief of Defence Staff Gen. Rick Hillier signed the original prisoner handover agreement back in 2005.

However, the deal was criticized because it had no clause to allow Canada to follow up on the treatment of detainees handed over.