Giuliano Zaccardelli, commissioner of the Royal Canadian Mounted Police, said on Thursday he was "truly sorry" for "the nightmare" Maher Arar experienced and for "whatever part" the federal police actions "may have contributed to the terrible injustices" his family endured.
"It is true that the early days after 9/11 were confusing and challenging. Of course this doesn't excuse or allow us to avoid facing head-on the ramifications of that time," he told a parliamentary standing committee on public safety and national security.
Arar was stopped in September 2002 while he was travelling through New York, on his way to Canada from a trip to Tunisia, and was deported to Syria where he was jailed and tortured for more than a year, said a Canadian report released mid-September.
Syria denies the torture claims and Washington has refused to accept blame for any wrongdoing in the case.
The 822-page report, which cleared Arar of terrorism ties, stated that US authorities had likely relied on faulty intelligence provided by Canadian police to hold and deport the 36-year-old software engineer to Syria.
The Mounties had provided "inaccurate" information to US authorities saying Arar was an "Islamic extremist" linked to Osama bin Laden's al-Qaeda terrorist group.
However, Zaccardelli said on Thursday that US authorities were advised of the mistake while Arar was still in US custody in New York.
"When Mr Arar was in New York City, we clearly communicated with the Americans that there was false information there and we tried to correct that false information," he testified.
"I have no information, no indication as to why the Americans took the decision ... to detain him and send him to Syria," Zaccardelli said.
"We have attempted to get that information. We have not gotten that," despite closer US-Canada security ties since 2002, he later said.
Alberto Gonzales, the US attorney-general, said last week: "We were not responsible for his removal to Syria," and added it was not a rendition, the transfer of alleged terrorists to CIA custody, as some alleged.
"It was a deportation," he explained. Arar is a Canadian citizen born in Syria.