Speaking to a congressional committee this morning, General Keith Alexander revealed that government surveillance efforts have thwarted “over 50” possible terrorist threats since 9/11, including plans to bomb the New York Stock Exchange and NYC subway system. Alexander, who heads the NSA, specifically noted that programs like the Verizon metadata collection may have been able to stop 9/11 if they had been in place at the time. He suggested that this program as well as “other intelligence” — which he did not disclose — assisted in foiling the 50 or so threats.

Alexander noted that the foiled plots occurred worldwide, not just domestically, although further details for most incidents aren't being publicly given at the moment. According to NBC News, a congressional committee will be briefed on classified details of all of the attacks this Wednesday.

The NSA director also took the opportunity to expand upon the agency's surveillance privileges under the often-cited 215 and 702 clauses of the Patriot Act and FISA Amendments Act. The NSA believes that abilities granted by 702 — which allows it to collect data on foreigners outside of the US — were necessary in the identification and arrest of Najibullah Zazi, who was found to have taken part in planning a NYC subway attack. However, the NSA's role in his capture — and the necessity of using 702 powers — remains in question. British intelligence had helped to identify Zazi even before the NSA began intercepting his email.

Constraints to the NSA's collection abilities under 215 — what allows the agency to gather telephone metadata like it does from Verizon — were detailed as well. Alexander said that there were "fewer than 300 unique [telephone] numbers that were used to initiate a query" within the NSA's database of metadata in all of 2012. However, additional queries may have been made based on the NSA's subsequent findings. When questioned on why the data requests couldn't have been made directly to companies, Alexander defended the NSA's policy of collecting and storing the data by espousing the agency's need to receive immediate results.

But while the NSA can only collect telephone metadata under 215, Alexander suggested that other agencies may be able to acquire broader information. He also confirmed that the NSA is able to make these queries without a warrant, though he suggested that oversight within the NSA itself helped to mitigate privacy concerns.

Alexander also backed the line that company after company has reported since appearing on the leaked PRISM slides, telling Congress that "the US government does not unilaterally collect information from the servers of US companies." It remains unclear exactly how the NSA gathers data from companies listed on the PRISM slides, however later information has suggested that it may be different from company to company, and may be an indirect form of access.