Toronto (search) is home to one of the largest Islamic communities in North America and is widely regarded as one of the world's most ethnically diverse cities.

But critics charge this tolerant and open society has also created an opening for terrorists who have their sights trained across the border at the United States.

"Al Qaeda (search) is here," said John Thompson, of the Mackenzie Institute (search), a Toronto-based think tank.

A Mackenzie study released last month found 15 of 80 identified international terrorist groups, including those responsible for homicide bombings in Israel, have a presence in Canada.

The report charges Usama bin Laden's terrorist network extends to at least 25 operatives in Canada and included Ahmed Ressam (search), who used America's northern neighbor as a base for his millennium bombing plot. Ressam was convicted for his conspiracy to bomb Los Angeles International Airport in December 1999.

Canada is certainly no stranger to terrorism. Until Sept. 11, 2001, the most deadly case of air terrorism in history originated in that country.

More than 300 people died aboard an Air India flight in 1985 en route from Montreal to Delhi when explosives planted in luggage detonated over the Atlantic Ocean. No one has been convicted in the killings, which remain under investigation.

The Air India bombing case, currently in court in British Columbia, is the Canadian equivalent of the 1989 Pam Am bombing over Lockerbie, Scotland, that killed 259 passengers traveling from Frankfurt to New York and 11 people on the ground.

Despite that history, however, critics say Canada’s open immigration and refugee policies could be creating a safe harbor for terrorists targeting the United States. Potential terrorists could be slipping under the radar of Canadian politicians wary of appearing intolerant, they say.

"It's political correctness. No political figure can really afford to be charged with racism by an interest group," Thompson said.

A senior administration official told Fox News the United States sees potential repercussions of Canada's leniency for security within U.S. borders. The United States needs Canada's cooperation to prevent future terrorist attacks.

And once terrorists make it across Canada's more lenient borders, it's all too easy for them to gain access to the United States, experts say.

"We need to harmonize our visa policies," said Asa Hutchinson, undersecretary for Borders and Transportation Security. "We've had great discussions with them. They have their own relationships with countries and their own policies, and we respect that, but they understand that’s a vulnerability that has to be looked at."

But Canada's solicitor general, the equivalent of the U.S. attorney general, clearly feels otherwise.

"Overall, antiterrorism cooperation with Canada remains excellent …The government of Canada has been a helpful and strong supporter of the United States in the fight against terrorism," according to a statement released from that country in regards to a recent U.S. State Department report.

Canadian officials say the Mackenzie report contains a number of inaccuracies, but the author is standing by his work.

"We've been lucky twice. We're lucky Ahmed Ressam did not actually get inside the United States with his load of explosives. We're also very lucky the 9/11 attackers did all directly enter the United States. We might not be lucky a third time," Thompson said.

Fox News’ Catherine Herridge and Darragh Worland contributed to this report.