He just can’t actually name any.

The director of the National Security Agency has defended government programs that monitor phone calls and emails, claiming they have disrupted ‘dozens’ of terrorist attacks.
NSA director Keith Alexander defended the programs as ‘critical’ at a Senate hearing on Wednesday – a week after a former contractor, Edward Snowden, leaked information about the snooping.
While he refused to discuss specifics, he did say federal data mining appeared to play a role in helping disrupt a recent plot to attack the New York subway system.

The bigger problem is that the NSA was conducting surveillance domestically on innocent Americans, basically a suspension of their Fourth Amendment rights.

General Alexander doesn’t believe that the terror problem began as an immigration problem.

You didn’t need NSA or its surveillance system to thwart the 9/11 hijackers. Most were in the United States on expired visas hiding in plain sight. Mohammed Atta was pulled over for a traffic violation and never showed in court:

On August 23, Atta’s driver license was revoked in absentia after he failed to show up in traffic court to answer the earlier citation for driving without a license.[77] On the same day, Israeli Mossad reportedly gave his name to the CIA as part of a list of 19 names they said were planning an attack in the near future.

We had systems in place to prevent this terror attack and they failed. Government didn’t need to mine metadata for this. Government simply needed to enforce our immigration laws. Atta and the others weren’t hiding—even after we were reportedly given from Mossad that one or more of these individuals were planning a future attack. Who knows how many more may be prevented as well, if only we were able to properly manage immigration by securing our borders and more selectively issuing visas.

It’s a weak excuse to lamely justify big, unaccountable government and its anti-4th Amendment actions when our current laws, enforced, would have sufficed.