FBI Director Christopher Wray delivered a full-throated defense of a powerful spy authority on Tuesday, invoking the Sept. 11, 2001, terrorist attacks and warning lawmakers that a watered-down surveillance program could leave the agency paralyzed to respond to threats.

“What could anybody possibly say to victims’ families if there was another attack that we could have prevented if we hadn’t given away the ability to effectively use a tool,” Wray told lawmakers on the Senate Judiciary Committee.

“Because let’s not fool ourselves. That’s what’s at stake with the reauthorization,” he said.

Wray escalated his rhetoric as Congress this week considers legislation that would add privacy protections for American information collected under Section 702 of the Foreign Intelligence Surveillance Act, which expires at the end of the year.

The House Judiciary Committee is set to consider a reauthorization bill Wednesday that would insert a warrant requirement when it comes to information on Americans, with certain exceptions. The House Intelligence Committee is scheduled to mark up its own reauthorization bill Thursday, which has not yet been released but is expected to have a different level of privacy provisions.

Section 702 allows the U.S. government to collect digital communications of foreigners located outside the country. But the program also sweeps up the communications of Americans and allows the FBI to search through data without a warrant using information such as an email address.

Wray’s testimony follows a letter to senators Monday from the Justice Department and the Office of the Director of National Intelligence that said Section 702 should not be allowed to lapse, and that a warrant requirement would “severely undercut” use of the authority to warn of terrorist attacks and malicious cyber activity.

The FBI director made the remarks to some of the senators who have insisted on more privacy protections and contend warrantless searches for information on Americans — known as U.S. person queries — are a clear violation of the Fourth Amendment.

Wray said stripping the FBI of its Section 702 authorities would be a form of “unilateral disarmament,” and the authority is key in finding out when a foreign terrorist organization directs an American-based operative to carry out an attack.

Wray’s comments drew a sharp rebuke from Sen. Mike Lee, R-Utah, who rejected the idea that adding a warrant requirement would equate to a unilateral disarmament.

“You have a lot of gall, sir. This is disgraceful,” Lee told Wray. “The Fourth Amendment requires more than that and you know it.”

Wray responded that no court that’s looked at Section 702 has found the authority to be unconstitutional.

Committee Chair Richard J. Durbin went to FBI headquarters this week for a demonstration on how the agency uses Section 702, but on Tuesday showed no signs of giving the agency a straight reauthorization.

The Illinois Democrat noted that the FBI has put in place changes to address compliance issues, but he said he’s still concerned about protecting the “communications of innocent Americans from warrantless surveillance.”

“I look forward to continuing to work with you to reauthorize 702 with the significant reforms we need to protect the privacy of innocent Americans,” Durbin said.

Wray testified that the warrant requirement would undercut the effectiveness of the spy authority.

“U.S. person queries, in particular, may provide the critical link that allows us to identify the intended target or build out the network of attackers so we can stop them before they strike and kill Americans,” he said.

Wray recalled being at FBI headquarters on Sept. 11, 2001, and said he’s spoken with the families of victims over the years.

“Before that attack, well-intentioned policymakers had made the choice to build a wall preventing access to national security information sitting in our and our partners’ holdings,” Wray warned, looking at the lawmakers.

“I bring that up because allowing 702 to lapse or amending it in a way that undermines its effectiveness would be akin to laying bricks to rebuild another pre-9/11 style wall,” he said.

Privacy advocates and lawmakers in both parties have raised concerns about the apparent breadth of the agency’s use of Section 702 and the agency’s documented compliance issues.

Over three years, starting in 2020, the FBI alone searched Section 702 databases for U.S. person information nearly 5 million times, according to the Privacy and Civil Liberties Oversight Board, an independent U.S. government board.

The Foreign Intelligence Surveillance Court has also found “persistent and widespread” compliance problems with the FBI’s searches under Section 702. The FBI had improperly searched foreign surveillance information using the last names of a U.S. senator and a state-level politician, according to a court opinion released earlier this year.

In the Monday letter to senators, the U.S. officials said the FBI had “implemented significant remedial measures” that quickly and dramatically improved compliance.

“Codifying these reforms would further buttress FBI’s compliance with the strict legal regime and regulatory framework that govern Section 702 activities and reduce the risk of the same reoccurrance of errors,” the letter states.