WASHINGTON – Four top U.S. intelligence officials testified before a U.S. Senate committee Monday morning, briefing senators and the public on security and intelligence changes implemented since Sept. 11, 2001.

Sen. Susan Collins, R-Maine, the senior Republican on the Senate Homeland Security and Governmental Affairs Committee, said she feared complacency now that six years have passed since the terrorist attacks.

“My concern is that our response may be in danger of flagging,” Collins said at the hearing in her opening remarks. “If we allow ourselves to become complacent, to revert to ‘September 10′ thinking, the next attack will not be due to a failure of imagination, but a failure of resolve.”

All four officials who testified addressed Collins’ concerns and emphasized the continued vigilance from their respective departments.

“While we have successfully raised our barrier against terrorist attacks, the fact remains that we are still a nation at risk,” said Michael Chertoff, secretary of the Department of Homeland Security.

Also testifying were National Intelligence Director Michael McConnell, FBI Director Robert Mueller and National Counterterrorism Center Director John Scott Redd.

The intelligence leaders said the disruption of terrorist plots against Fort Dix, N.J., and John F. Kennedy Airport in New York City earlier this year are examples of the effectiveness of their policies on intelligence sharing and increased analysis.

Collins said none of the testimony surprised her, but it was important because it allowed the public a window into how the intelligence world has changed since Sept. 11.

“It is a useful process for the American people to hear the analysis of our best minds in government,” Collins said in an interview after the hearing.

But while Collins strongly warned the top officials against relaxing in the fight against terrorism, she said in the interview that she is “taking issue” with the pending Western Hemisphere Travel Initiative that would require passports to cross the Canadian border. Collins said Mainers from Aroostook and Washington counties may cross the border frequently to visit family or commute back and forth every day for work, and if passports were required it would be “a very expensive endeavor.” She has filed legislation that would study the feasibility of using a driver’s license for identification rather than requiring the more expensive passport.

The current cost of acquiring a U.S. passport is about $100, according to the Department of State.

“We need a system that keeps our enemies out, but allows our friends in,” Collins said. “It’s a very challenging problem.”

The Western Hemisphere Travel Initiative is a change in travel document requirements being implemented by the Department of Homeland Security as the result of a recommendation by the 9-11 Commission. Passports will be required for U.S. citizens traveling by air to Canada, Mexico, the Caribbean and Bermuda beginning Oct. 1, 2007. Passports eventually will be required for all land and sea travelers to the same areas, but that deadline has not been established.

Ultimately, Collins said, there is more work to be done when it comes to homeland security.

“I believe we are clearly more safe than we were then – we have many more resources and intelligence than we had six years ago. But we are not yet safe,” she said.

When asked if there will ever be a time we can call ourselves safe, Collins said, “Not really.”