(NBC NEWS) -- We are learning more about the efforts of the national security agency at its controversial surveillance program.
Today government surveillance officials told a house committee, that its email and telephone programs have helped stop some 50 planned attacks here in the United States and abroad.
"These programs are immensely valuable for protecting our nation and securing the security of our allies," NSA director Keith Alexander told lawmakers.
Included in the effort, a plot to bomb the New York Stock Exchange.
The FBI says that assault was detected and stopped early in the planning stage.
Tuesday on CNBC, an senior exchange official said he was pleased to hear government efforts worked.
It's somewhat reassuring that the folk are out there doing the job - protecting us," said Gordon Charlop.
Officials say it was email from Yemen that put them onto the plot.
And there were more information on a plot to blow up the New York subway system.
In that case, officials say they tracked emails from Najibullah Zazi in Colorado to Al Qaeda in Pakistan.
They eventually tracked Zazi to New York where he was arrested before the attack could be carried out.
"And found bomb making components in backpacks. Zazi later confessed to a plot to bomb the us subway system with backpacks," said FBI Deputy Director Sean Joyce.
On Monday, President Obama was asked about the Zazi case.
"Now, we might have caught him some other way. We might have disrupted it because a new york cop saw he was suspicious," said the president. Adding... "But at the margins we are increasing our chances of preventing a catastrophe like that through these programs."
And Alexander told lawmakers he might have detected and stopped the 9/11 plot if he'd had then what NSA has now.