WASHINGTON -- In the wake of a new report showing how easily radioactive materials could be brought into the U.S. from Canada, senators jousted Thursday with a U.S. Border Patrol official over how the agency will better safeguard the border.

A Government Accountability Office probe of security lapses in "unmanned and unmonitored" areas between regular checkpoints found that investigators carrying large bags easily crossed the border illegally and unchallenged.

Senate Finance Committee Chairman Max Baucus, D-Mont., whose panel requested the investigation, asked at a hearing how many more border agents are needed.

"That's the multibillion-dollar question, sir," said Ronald Colburn, deputy chief of the Border Patrol. "We intend to bring on hundreds more, but the actual number we're still working on as we speak."

Baucus asked whether the agency will reassess the threat and redeploy personnel based on the vulnerability of the northern border.

"We already knew this," Colburn said. "This is not a surprise report to us."

The senator questioned why the agency hasn't been doing anything about it then, and Colburn retorted, "We are doing it."

Baucus accused Colburn of avoiding questions and not giving specifics.

"Frankly your testimony has not been satisfactory, it's not been candid," Baucus said. "I don't get that sense you really care about this. I don't get the sense you really deeply understand it."

But Colburn said he takes the issue very seriously and that the agency is adding manpower, unmanned aerial vehicles, aircraft, boats, and sensing systems.

"We have a national strategy and we depend heavily on the chiefs of those sectors along the northern border to give us their requirement to incrementally bring operational control to the entire U.S. border," he said. "We're not there. We're getting there."

Colburn added, "I'll say it quite frankly: GAO, tell me something I don't know. We know this, we've been telling you this. Our request to you is please continue to support us and resource us so we can gain operational control of the border."

Resources are first deployed to the most vulnerable, highest-risk areas, Colburn said. He added that less than 1 percent of all detected illegal active is on the Canadian border and 99 percent is on the border with Mexico.

GAO officials played a video showing a man with a bright red bag strolling across a field at midday, a potential terrorist bringing contraband from Canada into the United States with no problem.

"As you'll see in this video, it is quite easy to transport enough radioactive material to cause significant damage in a dirty bomb," testified John Cooney, GAO assistant director of forensic audits and special investigations.

The red bag, part of an investigation into border security by the GAO, contained counterfeit credit cards, detonators, radioactive material and narcotics, Cooney told a hearing of the Senate Finance Committee.

"The security on that border is really not increased too much since the French and Indian War, frankly," Cooney said.

GAO investigators visited four locations along the Canadian border and three along the Mexican border and simulated smuggling in radioactive material.

In two of the northern areas, there was no U.S. Border Patrol response. In another, a citizen alerted authorities but agents were not able to locate the investigators. In a fourth area, a Border Patrol agent spoke with the GAO investigators but let them go without verifying their identity.

"It's no surprise to anybody there are significant vulnerabilities to terrorists or criminals entering the United States from Canada," testified Greg Kutz, GAO managing director for forensic audits and special investigations.

About 1,000 agents work the northern border, compared to nearly 12,000 on the southern border. But only about a quarter of those are on duty at any one time, leaving about 250 agents to cover roughly 4,000 miles.

The GAO found the southern border to be "substantially more secure" than the northern border, Kutz said. Investigators near Mexico saw Border Patrol vehicles, Army National Guard units, helicopters and unmanned aerial vehicles, he said.

Kutz said the agency has made progress in personnel and technology but still has "insufficient resources to possibly meet the mission."

Colburn confirmed the GAO findings. "We agree that the border is not as secure as should be, as it needs to be," he said.

But he added, "In addressing some of the GAO findings, I will say, you don't always see us when we always see you."

He also said the agency has "come a long way." He noted that another 200 Border Patrol agents and five air bases are being added to the northern border now, and that the agency plans to add an undetermined number of agents in the future.