When authorities earlier this year discovered a 2,400-foot tunnel connecting a warehouse in Tijuana, Mexico, with another in the United States, Rep. Ed Royce, the California Republican who chairs the subcommittee on international terrorism, went to take a firsthand look.

He marveled at the "capital investment" made by its creators and pondered the security threat they might pose beyond trafficking in contraband like illicit drugs.

I asked Royce this week if he believes there is a risk that the sort of smugglers who built the tunnel could be exploited by al Qaeda to gain entry into the United States. "Absolutely," he said.

"We are playing with fire unless we move with the resources we have at our disposal to close down illegal immigration and to close down this measure of criminal activity at our border," said Royce. "We have not done it post-9/11, and this is threatening to our national security."

To bolster his analysis, Royce points to testimony given last year in the Senate Intelligence Committee by retired Coast Guard Admiral James Loy, who was then deputy secretary of Homeland Security. "Several al Qaeda leaders believe operatives can pay their way into the country through Mexico, and also believe illegal entry is more advantageous than legal entry for operational security reasons," Loy testified.

When I asked Royce if he believes Loy's testimony is credible, he said, "That's correct, and we also have heard that al Qaeda agents have themselves discussed coming into the country illegally to carry out operations in the United States."

Royce does not believe the Senate immigration bill is a proportionate response to this threat. In fact, he is convinced it will make things worse. "It is a response that makes it easier to carry out an attack," he said. "It's a response that ties the hands of law enforcement in ways they were not tied before."

Royce cited specific security defects in the bill, including Section 117, which requires every level of government in the United States to consult with every level of government in Mexico before any fencing or other structures can be built on the U.S. side of the border. "Federal, state, and local representatives in the United States shall consult with their counterparts in Mexico concerning the construction of additional fencing and related border security structures along the international border between the United States and Mexico before the commencement of any such construction," it says.

"This, to us, represents a key issue of sovereignty," said Royce. "Who does control the U.S. borders? Is it the government of the United States, or do our neighbors control it?"

Section 240D authorizes state and local law enforcement to "arrest, detain, or transfer to federal custody an alien for purposes of assisting in the enforcement of criminal provisions of the immigration laws." But, according to Royce and Kris Kobach (a professor at the University of Missouri law school in Kansas City, Mo., who served in the Justice Department as chief adviser to former Attorney General John Ashcroft on immigration issues), this language narrows the existing power of state and local police, who may currently detain aliens for both criminal and civil violations of immigration law.

"Specifically," says Royce, "the legislation would prevent state and local law enforcement from having the opportunity to apprehend a potential terrorist who had overstayed his visa."

"If it were enacted, the Senate bill would prevent state and local police from making arrests for civil violations of immigration law -- which are precisely the sorts of violations that five of the 9/11 terrorists committed," Kobach told me. "The Senate bill would tie the hands of state and local police, even though they are the law enforcement officers most likely to come into contact with terrorists operating on U.S. soil."

In a recent article for the Albany Law Review, Kobach detailed how four al Qaeda hijackers -- including three of the four suspected pilots -- were stopped by local law enforcement for traffic violations and were let go, even though they could have been arrested for civil violations of immigration law.

Not everyone agrees with Kobach's interpretation that Section 240D denies local authorities the power to arrest aliens for civil violations of immigration law. A Senate Judiciary Committee aide told me: "The committee provided a statutory basis for state and local authority on criminal violations. The committee did not address the issue of civil violations, thus maintaining current law."

Those who wish to defend the Senate bill will be given the chance to do so on a national stage. Royce is taking his subcommittee to the border in California and Texas on July 5 and 7 to hold hearings on what the Senate has done. "I want the American public to understand something that I think many of the senators did not understand," said Royce, "and that is what is really in the Senate bill and what would the consequences of enactment be for border security."