Top law enforcement and intelligence officials fiercely defended the Obama administration’s sweeping surveillance programs on Capitol Hill Thursday, emphasizing their legality, their record of success in thwarting terrorist attacks and the many opportunities lawmakers have had over the years to alter the programs that some are now criticizing as too intrusive.

FBI Director Robert S. Mueller III and Gen. Keith B. Alexander, director of the National Security Agency, stood up to both Republican and Democratic critics in separate hearings about the programs, whose broad monitoring of telephone calls and Internet use around the world was exposed last week by a former NSA contract employee, setting off a national debate about the delicate balance between security and privacy in the post-9/11 United States. Meanwhile, intelligence officials briefed senators on the programs.

In what was likely his last appearance before the House Judiciary Committee, Mueller stressed the administration’s line on the controversial telephone and Internet surveillance programs: Lawmakers knew about the programs and had years’ worth of opportunities to change them. He also suggested that if the intelligence community had its present surveillance authorities prior to 9/11, it may have been able to thwart the attacks that occurred that day.

Mueller, who took over the FBI a week before the 9/11 attacks, is expected to step down soon. President Barack Obama is expected to nominate James B. Comey, a senior Justice Department official in the George W. Bush administration, to replace him.

Rep. John Conyers Jr. of Michigan, the ranking Democrat on the committee, sparred with Mueller, contending that lawmakers’ knowledge of the programs did not make them complicit in the surveillance.

“The mere fact that some members may have been briefed in a classified setting does not indicate our approval” of these programs, Conyers said. “It’s my fear that we’re on the verge of becoming a surveillance state.”

Alexander told reporters Thursday that the intelligence community has used the surveillance programs to disrupt “dozens” of terrorist plots before they could harm Americans.

Some of the members in the briefing, however, came away with more modest numbers.

“At this point, I think we can say that at least this program has thwarted 10 possible terrorist attacks,” said C.A. Dutch Ruppersberger of Maryland, the ranking Democrat on the panel.

Mueller defended the legality the intelligence community’s actions, saying the goal has been to balance privacy with security.

“We recognize that the American people expect the FBI and its intelligence partners to protect privacy interests,” he said.

The FBI director said the disclosures made by Edward Snowden, the NSA contractor who has admitted to leaking details about the NSA surveillance, did “significant harm” to national security. Although Mueller would not comment on details of the bureau’s ongoing investigation into Snowden, he said the FBI is doing all it can to hold him responsible.

Senate Intelligence Chairwoman Dianne Feinstein, D-Calif., told reporters that she is looking at moving legislation that would “limit or prevent” contractors like Snowden from “handling highly classified technical data.”

Several members of the Judiciary panel, however, were skeptical of the administration’s justifications for the broad scope of its surveillance activities.

Wisconsin Republican Jim Sensenbrenner, who had a hand in authoring the Patriot Act, noted that the authority within the law that the NSA is using was intended to allow for data collection on foreigners who are the targets of an authorized terrorism investigation, along with any U.S. citizens who are in touch with them. He questioned how that provision could be used as a means for gathering information on Americans who don’t fall into that category.

Mueller responded that the administration believes that the wide swathes of data it has been collecting meet relevance requirements within that section of the law, a point which several members challenged.

The reasoning makes a “mockery” of the relevance requirement, said New York Democrat Jerrold Nadler. “If everything in the world is relevant, then there’s no meaning to that word.”

Mueller also made the point that federal agencies are gathering only the dates and times of calls and the locations from which they were made. The Supreme Court, the director said, has held that such records do not fall under the Fourth Amendment’s privacy protections.

“We tend to confuse that which is covered by the Fourth Amendment with that which is not covered by the Fourth Amendment,” he said.

Nadler, however, noted that the Supreme Court’s ruling was from a case in the 1970s involving phone bills, and might not apply to modern metadata.

During an exchange with Conyers, Mueller noted that prior to the 9/11 attacks, intelligence officials had been monitoring Saudi Arabian national Khalid al-Mihdhar — one of the hijackers in the attack on the Pentagon — but lost track of him. Agents were also aware of an al-Qaida safehouse in Yemen with a phone number, but didn’t know who might have called. With today’s authorities, Mueller said, the agencies could have discovered that al-Mihdhar had called the safehouse from San Diego.

Using that information to detain al-Mihdhar or any of his co-conspirators could have hurt their plot, Mueller said. “The simple fact of their detention could have derailed the plan,” he said.

Alexander said he tried to address lawmakers’ concerns about greater transparency to reassure Americans about the surveillance programs. He described for lawmakers a review process under way intended to find ways to provide the public with more information about the surveillance, so that “they can understand the value of these programs in saving American lives.”

He also said the government owes U.S. citizens statistics about how effective the programs are.

“And I think when the American people hear that, they’re going stop and say, ‘Wait, the information we’re getting is incorrect,’” he said. “So I would just tell the American people, ‘Let’s take a step back, look at what’s going on, the oversight and compliance, and then let’s have this discussion.’”

Following the Senate briefing, Mike Johanns, R-Neb., said he feels more comfortable with the surveillance.

“I wouldn’t say [my concerns] were 100 percent resolved, but I think some very good steps were taken,” he said.

Humberto Sanchez contributed to this report.