"I do think it's important that we get this right, and I want the American people to know that we're trying to be transparent here, protect civil liberties and privacy but also the security of this country," Alexander told a Senate panel.

Half a world away, Edward Snowden, the former contractor who fled to Hong Kong and leaked details of the programs, said he would fight any U.S. attempts to extradite him. U.S. law enforcement officials are building a case against him but have yet to bring charges.

"I am not here to hide from justice; I am here to reveal criminality," Snowden said in an interview with the local South China Morning Post.

Alexander described the steps the government takes once it suspects a terrorist organization is about to act — all within the laws approved by Congress and under stringent oversight from the courts. He said the programs led to "disrupting or contributing to the disruption of terrorist attacks" but didn't give details.

Alexander warned that revelations about the secret programs have eroded agency capabilities and, as a result, the U.S. and its allies won't be as safe as they were two weeks ago.

"Some of these are still going to be classified and should be, because if we tell the terrorists every way that we're going to track them, they will get through and Americans will die," he said.

Alexander said he was seriously concerned that Snowden, a former employee with Booz Allen Hamilton, had access to key parts of the NSA network, a development that demands a closer examination of how well the agency oversees contract employees.

Alexander said Snowden was a system administrator who didn't have visibility into the whole NSA network but could access key portions of it.

The director was questioned at length by senators seeking information on exactly how much data the NSA gathers through programs to collect millions of telephone records and keep tabs on Internet activity, as well as the legal backing for the activities.

Members of the House and Senate Intelligence panels and key leaders have expressed their support for the operations as a valid tool in the terrorism fight. But rank-and-file lawmakers who haven't been privy to the details expressed concerns and bewilderment.

Sen. Mike Johanns asked the NSA director whether the government could check and see what an individual is searching for through Google, or sending in email.

Alexander said once an individual has been identified, the issue is referred to the FBI.

"The FBI will then look at that and say what more do we need to now look at that individual themselves. So there are issues and things that they would then look at. It's passed to them," Alexander said.

"So the answer to the question is yes," Johanns said.

"Yes, you could. I mean, you can get a court order to do that," Alexander said.

Congressional leaders and intelligence committee members have been routinely briefed about the spy programs, officials said, and Congress has at least twice renewed laws approving them.

But the disclosure of their sheer scope stunned some lawmakers, shocked allies from nations with strict privacy protections, and emboldened civil liberties advocates who long have accused the government of being too invasive in the name of national security.

Sen. Rand Paul said he plans on Thursday to announce "legal action against government surveillance and the National Security Agency's overreach of power," his political office said.

Recent polling on the issue found Americans troubled by the intrusion but perhaps willing to give the government even more room in its efforts to fight terrorism.

A new poll by CBS News and The New York Times found that 58 percent disapprove of the government collecting phone records of all Americans. Yet it also found that 59 percent think the government has struck the right balance or not gone far enough.

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Associated Press writers Donna Cassata, Connie Cass, Lara Jakes, Kimberly Dozier, Frederic Frommer, Alan Fram, Andrew Miga and Pete Yost contributed to this report.