Canada has increased security at its airports and borders in the wake of the Sept. 11, 2001, attacks on the United States, but experts say it is hard to gauge the strength of the new measures five years later.

"It's very hard to measure whether Canada's safer now than it was on, say Sept. 10, 2001," said Barry Cooper, a political science professor at the University of Calgary. "If there's one thing about terrorists, they can find out about your security [measures] and work around them."

The Liberal government at the time of the attacks promised nearly $8 billion in spending to boost security at Canada's airports and elsewhere.

"If laws need to be changed, they will be,"Prime MinisterJean Chrétien vowed at that time. "If security has to be increased to protect Canadians, it will be."

The new security measures include tougher screening of luggage at airports, an increase in the number of guards along the Canada-U.S. border,additional agents for CSIS and expanded powers for police to crack down on suspected militants plotting attacks on Canadian soil.

Canadians not invulnerable

John Manley, who was the minister put in charge of the security buildup,said Canadians should not feel invulnerable to extremist threats.

"I don't think a plane will fly into a building again because we've barricaded cockpits and other things," Manley, now out of politics,told CBC News. "I do fear there are other ways that the civilian population could be terrified and harmed by people willing to do it."

An example of that came last month, when British police arrested two dozen people inthe London area, accusing them of plotting to blow up planes heading for the United States using gels and liquids that could be combined in mid-flight to forman explosive material.

Soon after, Transport Canada imposed increased restrictions on travellers at Canadian airports. Among other things,passengers were no longer allowed to carry water, shampoo or other liquids and gels in their carry-on luggage.

Conservatives add money to security budget

The Conservative government, whichtook office in January,has kept up the former Liberal government's emphasis on improving security, pledging another $1.4 billion in its spring budget.

"You can never be 100-per-cent safe; there's no system in the world that can guarantee that," said Public Safety Minister Stockwell Day. "But I can tell you our levels of safety and security are the highest anywhere in the world."

Prime Minister Stephen Harper, who was Opposition leader in 2002,paid tribute Monday evening to the victims of the attacks while making the case for the necessity of the Canadian operation in Afghanistan.

Surrounded by relatives of some of the 24 Canadians killed in the U.S. on Sept. 11, 2001, Harper stressed the importance of Canada's role in Afghanistan, calling the Taliban a source of the terror that led to the deadly attacks.

More than 2,000 Canadiansare stationed in Kandahar; in May,Parliament voted in favour of committing the Afghan mission to 2009.